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• Synthesizing Sources | Examples & Synthesis Matrix

Synthesizing Sources | Examples & Synthesis Matrix

Published on July 4, 2022 by Eoghan Ryan . Revised on May 31, 2023.

Synthesizing sources involves combining the work of other scholars to provide new insights. It’s a way of integrating sources that helps situate your work in relation to existing research.

Synthesizing sources involves more than just summarizing . You must emphasize how each source contributes to current debates, highlighting points of (dis)agreement and putting the sources in conversation with each other.

You might synthesize sources in your literature review to give an overview of the field or throughout your research paper when you want to position your work in relation to existing research.

Table of contents

Example of synthesizing sources, how to synthesize sources, synthesis matrix, other interesting articles, frequently asked questions about synthesizing sources.

Let’s take a look at an example where sources are not properly synthesized, and then see what can be done to improve it.

This paragraph provides no context for the information and does not explain the relationships between the sources described. It also doesn’t analyze the sources or consider gaps in existing research.

Research on the barriers to second language acquisition has primarily focused on age-related difficulties. Building on Lenneberg’s (1967) theory of a critical period of language acquisition, Johnson and Newport (1988) tested Lenneberg’s idea in the context of second language acquisition. Their research seemed to confirm that young learners acquire a second language more easily than older learners. Recent research has considered other potential barriers to language acquisition. Schepens, van Hout, and van der Slik (2022) have revealed that the difficulties of learning a second language at an older age are compounded by dissimilarity between a learner’s first language and the language they aim to acquire. Further research needs to be carried out to determine whether the difficulty faced by adult monoglot speakers is also faced by adults who acquired a second language during the “critical period.”

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To synthesize sources, group them around a specific theme or point of contention.

As you read sources, ask:

• What questions or ideas recur? Do the sources focus on the same points, or do they look at the issue from different angles?
• How does each source relate to others? Does it confirm or challenge the findings of past research?
• Where do the sources agree or disagree?

Once you have a clear idea of how each source positions itself, put them in conversation with each other. Analyze and interpret their points of agreement and disagreement. This displays the relationships among sources and creates a sense of coherence.

Consider both implicit and explicit (dis)agreements. Whether one source specifically refutes another or just happens to come to different conclusions without specifically engaging with it, you can mention it in your synthesis either way.

Synthesize your sources using:

• Topic sentences to introduce the relationship between the sources
• Signal phrases to attribute ideas to their authors
• Transition words and phrases to link together different ideas

To more easily determine the similarities and dissimilarities among your sources, you can create a visual representation of their main ideas with a synthesis matrix . This is a tool that you can use when researching and writing your paper, not a part of the final text.

In a synthesis matrix, each column represents one source, and each row represents a common theme or idea among the sources. In the relevant rows, fill in a short summary of how the source treats each theme or topic.

This helps you to clearly see the commonalities or points of divergence among your sources. You can then synthesize these sources in your work by explaining their relationship.

If you want to know more about ChatGPT, AI tools , citation , and plagiarism , make sure to check out some of our other articles with explanations and examples.

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Synthesizing sources means comparing and contrasting the work of other scholars to provide new insights.

It involves analyzing and interpreting the points of agreement and disagreement among sources.

You might synthesize sources in your literature review to give an overview of the field of research or throughout your paper when you want to contribute something new to existing research.

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a thesis, dissertation , or research paper , in order to situate your work in relation to existing knowledge.

Topic sentences help keep your writing focused and guide the reader through your argument.

In an essay or paper , each paragraph should focus on a single idea. By stating the main idea in the topic sentence, you clarify what the paragraph is about for both yourself and your reader.

At college level, you must properly cite your sources in all essays , research papers , and other academic texts (except exams and in-class exercises).

Add a citation whenever you quote , paraphrase , or summarize information or ideas from a source. You should also give full source details in a bibliography or reference list at the end of your text.

The exact format of your citations depends on which citation style you are instructed to use. The most common styles are APA , MLA , and Chicago .

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When you write a literature review or essay, you have to go beyond just summarizing the articles you’ve read – you need to synthesize the literature to show how it all fits together (and how your own research fits in).

Synthesizing simply means combining. Instead of summarizing the main points of each source in turn, you put together the ideas and findings of multiple sources in order to make an overall point.

At the most basic level, this involves looking for similarities and differences between your sources. Your synthesis should show the reader where the sources overlap and where they diverge.

Unsynthesized Example

Franz (2008) studied undergraduate online students. He looked at 17 females and 18 males and found that none of them liked APA. According to Franz, the evidence suggested that all students are reluctant to learn citations style. Perez (2010) also studies undergraduate students. She looked at 42 females and 50 males and found that males were significantly more inclined to use citation software ( p < .05). Findings suggest that females might graduate sooner. Goldstein (2012) looked at British undergraduates. Among a sample of 50, all females, all confident in their abilities to cite and were eager to write their dissertations.

Synthesized Example

Studies of undergraduate students reveal conflicting conclusions regarding relationships between advanced scholarly study and citation efficacy. Although Franz (2008) found that no participants enjoyed learning citation style, Goldstein (2012) determined in a larger study that all participants watched felt comfortable citing sources, suggesting that variables among participant and control group populations must be examined more closely. Although Perez (2010) expanded on Franz’s original study with a larger, more diverse sample…

Step 1: Organize your sources

After collecting the relevant literature, you’ve got a lot of information to work through, and no clear idea of how it all fits together.

Before you can start writing, you need to organize your notes in a way that allows you to see the relationships between sources.

One way to begin synthesizing the literature is to put your notes into a table. Depending on your topic and the type of literature you’re dealing with, there are a couple of different ways you can organize this.

Summary table

A summary table collates the key points of each source under consistent headings. This is a good approach if your sources tend to have a similar structure – for instance, if they’re all empirical papers.

Each row in the table lists one source, and each column identifies a specific part of the source. You can decide which headings to include based on what’s most relevant to the literature you’re dealing with.

For example, you might include columns for things like aims, methods, variables, population, sample size, and conclusion.

For each study, you briefly summarize each of these aspects. You can also include columns for your own evaluation and analysis.

The summary table gives you a quick overview of the key points of each source. This allows you to group sources by relevant similarities, as well as noticing important differences or contradictions in their findings.

Synthesis matrix

A synthesis matrix is useful when your sources are more varied in their purpose and structure – for example, when you’re dealing with books and essays making various different arguments about a topic.

Each column in the table lists one source. Each row is labeled with a specific concept, topic or theme that recurs across all or most of the sources.

Then, for each source, you summarize the main points or arguments related to the theme.

The purposes of the table is to identify the common points that connect the sources, as well as identifying points where they diverge or disagree.

Step 2: Outline your structure

Now you should have a clear overview of the main connections and differences between the sources you’ve read. Next, you need to decide how you’ll group them together and the order in which you’ll discuss them.

For shorter papers, your outline can just identify the focus of each paragraph; for longer papers, you might want to divide it into sections with headings.

There are a few different approaches you can take to help you structure your synthesis.

If your sources cover a broad time period, and you found patterns in how researchers approached the topic over time, you can organize your discussion chronologically .

That doesn’t mean you just summarize each paper in chronological order; instead, you should group articles into time periods and identify what they have in common, as well as signalling important turning points or developments in the literature.

If the literature covers various different topics, you can organize it thematically .

That means that each paragraph or section focuses on a specific theme and explains how that theme is approached in the literature.

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If you’re drawing on literature from various different fields or they use a wide variety of research methods, you can organize your sources methodologically .

That means grouping together studies based on the type of research they did and discussing the findings that emerged from each method.

If your topic involves a debate between different schools of thought, you can organize it theoretically .

That means comparing the different theories that have been developed and grouping together papers based on the position or perspective they take on the topic, as well as evaluating which arguments are most convincing.

Step 3: Write paragraphs with topic sentences

What sets a synthesis apart from a summary is that it combines various sources. The easiest way to think about this is that each paragraph should discuss a few different sources, and you should be able to condense the overall point of the paragraph into one sentence.

This is called a topic sentence , and it usually appears at the start of the paragraph. The topic sentence signals what the whole paragraph is about; every sentence in the paragraph should be clearly related to it.

A topic sentence can be a simple summary of the paragraph’s content:

“Early research on [x] focused heavily on [y].”

For an effective synthesis, you can use topic sentences to link back to the previous paragraph, highlighting a point of debate or critique:

“Several scholars have pointed out the flaws in this approach.” “While recent research has attempted to address the problem, many of these studies have methodological flaws that limit their validity.”

By using topic sentences, you can ensure that your paragraphs are coherent and clearly show the connections between the articles you are discussing.

As you write your paragraphs, avoid quoting directly from sources: use your own words to explain the commonalities and differences that you found in the literature.

Don’t try to cover every single point from every single source – the key to synthesizing is to extract the most important and relevant information and combine it to give your reader an overall picture of the state of knowledge on your topic.

Step 4: Revise, edit and proofread

Like any other piece of academic writing, synthesizing literature doesn’t happen all in one go – it involves redrafting, revising, editing and proofreading your work.

Checklist for Synthesis

•   Do I introduce the paragraph with a clear, focused topic sentence?
•   Do I discuss more than one source in the paragraph?
•   Do I mention only the most relevant findings, rather than describing every part of the studies?
•   Do I discuss the similarities or differences between the sources, rather than summarizing each source in turn?
•   Do I put the findings or arguments of the sources in my own words?
•   Is the paragraph organized around a single idea?
•   Is the paragraph directly relevant to my research question or topic?
•   Is there a logical transition from this paragraph to the next one?

Further Information

How to Synthesise: a Step-by-Step Approach

Help…I”ve Been Asked to Synthesize!

Learn how to Synthesise (combine information from sources)

How to write a Psychology Essay

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Synthesizing Sources

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When you look for areas where your sources agree or disagree and try to draw broader conclusions about your topic based on what your sources say, you are engaging in synthesis. Writing a research paper usually requires synthesizing the available sources in order to provide new insight or a different perspective into your particular topic (as opposed to simply restating what each individual source says about your research topic).

Note that synthesizing is not the same as summarizing.  

• A summary restates the information in one or more sources without providing new insight or reaching new conclusions.
• A synthesis draws on multiple sources to reach a broader conclusion.

There are two types of syntheses: explanatory syntheses and argumentative syntheses . Explanatory syntheses seek to bring sources together to explain a perspective and the reasoning behind it. Argumentative syntheses seek to bring sources together to make an argument. Both types of synthesis involve looking for relationships between sources and drawing conclusions.

In order to successfully synthesize your sources, you might begin by grouping your sources by topic and looking for connections. For example, if you were researching the pros and cons of encouraging healthy eating in children, you would want to separate your sources to find which ones agree with each other and which ones disagree.

After you have a good idea of what your sources are saying, you want to construct your body paragraphs in a way that acknowledges different sources and highlights where you can draw new conclusions.

As you continue synthesizing, here are a few points to remember:

• Don’t force a relationship between sources if there isn’t one. Not all of your sources have to complement one another.
• Do your best to highlight the relationships between sources in very clear ways.
• Don’t ignore any outliers in your research. It’s important to take note of every perspective (even those that disagree with your broader conclusions).

Example Syntheses

Below are two examples of synthesis: one where synthesis is NOT utilized well, and one where it is.

Parents are always trying to find ways to encourage healthy eating in their children. Elena Pearl Ben-Joseph, a doctor and writer for KidsHealth , encourages parents to be role models for their children by not dieting or vocalizing concerns about their body image. The first popular diet began in 1863. William Banting named it the “Banting” diet after himself, and it consisted of eating fruits, vegetables, meat, and dry wine. Despite the fact that dieting has been around for over a hundred and fifty years, parents should not diet because it hinders children’s understanding of healthy eating.

In this sample paragraph, the paragraph begins with one idea then drastically shifts to another. Rather than comparing the sources, the author simply describes their content. This leads the paragraph to veer in an different direction at the end, and it prevents the paragraph from expressing any strong arguments or conclusions.

An example of a stronger synthesis can be found below.

Parents are always trying to find ways to encourage healthy eating in their children. Different scientists and educators have different strategies for promoting a well-rounded diet while still encouraging body positivity in children. David R. Just and Joseph Price suggest in their article “Using Incentives to Encourage Healthy Eating in Children” that children are more likely to eat fruits and vegetables if they are given a reward (855-856). Similarly, Elena Pearl Ben-Joseph, a doctor and writer for Kids Health , encourages parents to be role models for their children. She states that “parents who are always dieting or complaining about their bodies may foster these same negative feelings in their kids. Try to keep a positive approach about food” (Ben-Joseph). Martha J. Nepper and Weiwen Chai support Ben-Joseph’s suggestions in their article “Parents’ Barriers and Strategies to Promote Healthy Eating among School-age Children.” Nepper and Chai note, “Parents felt that patience, consistency, educating themselves on proper nutrition, and having more healthy foods available in the home were important strategies when developing healthy eating habits for their children.” By following some of these ideas, parents can help their children develop healthy eating habits while still maintaining body positivity.

In this example, the author puts different sources in conversation with one another. Rather than simply describing the content of the sources in order, the author uses transitions (like "similarly") and makes the relationship between the sources evident.

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What Synthesis Methodology Should I Use? A Review and Analysis of Approaches to Research Synthesis

Kara schick-makaroff.

1 Faculty of Nursing, University of Alberta, Edmonton, AB, Canada

Marjorie MacDonald

2 School of Nursing, University of Victoria, Victoria, BC, Canada

Marilyn Plummer

3 College of Nursing, Camosun College, Victoria, BC, Canada

Judy Burgess

4 Student Services, University Health Services, Victoria, BC, Canada

Wendy Neander

Associated data, additional file 1.

When we began this process, we were doctoral students and a faculty member in a research methods course. As students, we were facing a review of the literature for our dissertations. We encountered several different ways of conducting a review but were unable to locate any resources that synthesized all of the various synthesis methodologies. Our purpose is to present a comprehensive overview and assessment of the main approaches to research synthesis. We use ‘research synthesis’ as a broad overarching term to describe various approaches to combining, integrating, and synthesizing research findings.

We conducted an integrative review of the literature to explore the historical, contextual, and evolving nature of research synthesis. We searched five databases, reviewed websites of key organizations, hand-searched several journals, and examined relevant texts from the reference lists of the documents we had already obtained.

We identified four broad categories of research synthesis methodology including conventional, quantitative, qualitative, and emerging syntheses. Each of the broad categories was compared to the others on the following: key characteristics, purpose, method, product, context, underlying assumptions, unit of analysis, strengths and limitations, and when to use each approach.

Conclusions

The current state of research synthesis reflects significant advancements in emerging synthesis studies that integrate diverse data types and sources. New approaches to research synthesis provide a much broader range of review alternatives available to health and social science students and researchers.

1. Introduction

Since the turn of the century, public health emergencies have been identified worldwide, particularly related to infectious diseases. For example, the Severe Acute Respiratory Syndrome (SARS) epidemic in Canada in 2002-2003, the recent Ebola epidemic in Africa, and the ongoing HIV/AIDs pandemic are global health concerns. There have also been dramatic increases in the prevalence of chronic diseases around the world [1] – [3] . These epidemiological challenges have raised concerns about the ability of health systems worldwide to address these crises. As a result, public health systems reform has been initiated in a number of countries. In Canada, as in other countries, the role of evidence to support public health reform and improve population health has been given high priority. Yet, there continues to be a significant gap between the production of evidence through research and its application in practice [4] – [5] . One strategy to address this gap has been the development of new research synthesis methodologies to deal with the time-sensitive and wide ranging evidence needs of policy makers and practitioners in all areas of health care, including public health.

As doctoral nursing students facing a review of the literature for our dissertations, and as a faculty member teaching a research methods course, we encountered several ways of conducting a research synthesis but found no comprehensive resources that discussed, compared, and contrasted various synthesis methodologies on their purposes, processes, strengths and limitations. To complicate matters, writers use terms interchangeably or use different terms to mean the same thing, and the literature is often contradictory about various approaches. Some texts [6] , [7] – [9] did provide a preliminary understanding about how research synthesis had been taken up in nursing, but these did not meet our requirements. Thus, in this article we address the need for a comprehensive overview of research synthesis methodologies to guide public health, health care, and social science researchers and practitioners.

Research synthesis is relatively new in public health but has a long history in other fields dating back to the late 1800s. Research synthesis, a research process in its own right [10] , has become more prominent in the wake of the evidence-based movement of the 1990s. Research syntheses have found their advocates and detractors in all disciplines, with challenges to the processes of systematic review and meta-analysis, in particular, being raised by critics of evidence-based healthcare [11] – [13] .

Our purpose was to conduct an integrative review of the literature to explore the historical, contextual, and evolving nature of research synthesis [14] – [15] . We synthesize and critique the main approaches to research synthesis that are relevant for public health, health care, and social scientists. Research synthesis is the overarching term we use to describe approaches to combining, aggregating, integrating, and synthesizing primary research findings. Each synthesis methodology draws on different types of findings depending on the purpose and product of the chosen synthesis (see Additional File 1 ).

3. Method of Review

Based on our current knowledge of the literature, we identified these approaches to include in our review: systematic review, meta-analysis, qualitative meta-synthesis, meta-narrative synthesis, scoping review, rapid review, realist synthesis, concept analysis, literature review, and integrative review. Our first step was to divide the synthesis types among the research team. Each member did a preliminary search to identify key texts. The team then met to develop search terms and a framework to guide the review.

Over the period of 2008 to 2012 we extensively searched the literature, updating our search at several time points, not restricting our search by date. The dates of texts reviewed range from 1967 to 2015. We used the terms above combined with the term “method* (e.g., “realist synthesis” and “method*) in the database Health Source: Academic Edition (includes Medline and CINAHL). This search yielded very few texts on some methodologies and many on others. We realized that many documents on research synthesis had not been picked up in the search. Therefore, we also searched Google Scholar, PubMed, ERIC, and Social Science Index, as well as the websites of key organizations such as the Joanna Briggs Institute, the University of York Centre for Evidence-Based Nursing, and the Cochrane Collaboration database. We hand searched several nursing, social science, public health and health policy journals. Finally, we traced relevant documents from the references in obtained texts.

We included works that met the following inclusion criteria: (1) published in English; (2) discussed the history of research synthesis; (3) explicitly described the approach and specific methods; or (4) identified issues, challenges, strengths and limitations of the particular methodology. We excluded research reports that resulted from the use of particular synthesis methodologies unless they also included criteria 2, 3, or 4 above.

Based on our search, we identified additional types of research synthesis (e.g., meta-interpretation, best evidence synthesis, critical interpretive synthesis, meta-summary, grounded formal theory). Still, we missed some important developments in meta-analysis, for example, identified by the journal's reviewers that have now been discussed briefly in the paper. The final set of 197 texts included in our review comprised theoretical, empirical, and conceptual papers, books, editorials and commentaries, and policy documents.

In our preliminary review of key texts, the team inductively developed a framework of the important elements of each method for comparison. In the next phase, each text was read carefully, and data for these elements were extracted into a table for comparison on the points of: key characteristics, purpose, methods, and product; see Additional File 1 ). Once the data were grouped and extracted, we synthesized across categories based on the following additional points of comparison: complexity of the process, degree of systematization, consideration of context, underlying assumptions, unit of analysis, and when to use each approach. In our results, we discuss our comparison of the various synthesis approaches on the elements above. Drawing only on documents for the review, ethics approval was not required.

We identified four broad categories of research synthesis methodology: Conventional, quantitative, qualitative, and emerging syntheses. From our dataset of 197 texts, we had 14 texts on conventional synthesis, 64 on quantitative synthesis, 78 on qualitative synthesis, and 41 on emerging syntheses. Table 1 provides an overview of the four types of research synthesis, definitions, types of data used, products, and examples of the methodology.

Although we group these types of synthesis into four broad categories on the basis of similarities, each type within a category has unique characteristics, which may differ from the overall group similarities. Each could be explored in greater depth to tease out their unique characteristics, but detailed comparison is beyond the scope of this article.

Additional File 1 presents one or more selected types of synthesis that represent the broad category but is not an exhaustive presentation of all types within each category. It provides more depth for specific examples from each category of synthesis on the characteristics, purpose, methods, and products than is found in Table 1 .

4.1. Key Characteristics

4.1.1. what is it.

Here we draw on two types of categorization. First, we utilize Dixon Woods et al.'s [49] classification of research syntheses as being either integrative or interpretive . (Please note that integrative syntheses are not the same as an integrative review as defined in Additional File 1 .) Second, we use Popay's [80] enhancement and epistemological models .

The defining characteristics of integrative syntheses are that they involve summarizing the data achieved by pooling data [49] . Integrative syntheses include systematic reviews, meta-analyses, as well as scoping and rapid reviews because each of these focus on summarizing data. They also define concepts from the outset (although this may not always be true in scoping or rapid reviews) and deal with a well-specified phenomenon of interest.

Interpretive syntheses are primarily concerned with the development of concepts and theories that integrate concepts [49] . The analysis in interpretive synthesis is conceptual both in process and outcome, and “the product is not aggregations of data, but theory” [49] , [p.12]. Interpretive syntheses involve induction and interpretation, and are primarily conceptual in process and outcome. Examples include integrative reviews, some systematic reviews, all of the qualitative syntheses, meta-narrative, realist and critical interpretive syntheses. Of note, both quantitative and qualitative studies can be either integrative or interpretive

The second categorization, enhancement versus epistemological , applies to those approaches that use multiple data types and sources [80] . Popay's [80] classification reflects the ways that qualitative data are valued in relation to quantitative data.

In the enhancement model , qualitative data adds something to quantitative analysis. The enhancement model is reflected in systematic reviews and meta-analyses that use some qualitative data to enhance interpretation and explanation. It may also be reflected in some rapid reviews that draw on quantitative data but use some qualitative data.

The epistemological model assumes that quantitative and qualitative data are equal and each has something unique to contribute. All of the other review approaches, except pure quantitative or qualitative syntheses, reflect the epistemological model because they value all data types equally but see them as contributing different understandings.

4.1.2. Data type

By and large, the quantitative approaches (quantitative systematic review and meta-analysis) have typically used purely quantitative data (i.e., expressed in numeric form). More recently, both Cochrane [81] and Campbell [82] collaborations are grappling with the need to, and the process of, integrating qualitative research into a systematic review. The qualitative approaches use qualitative data (i.e., expressed in words). All of the emerging synthesis types, as well as the conventional integrative review, incorporate qualitative and quantitative study designs and data.

4.1.3. Research question

Four types of research questions direct inquiry across the different types of syntheses. The first is a well-developed research question that gives direction to the synthesis (e.g., meta-analysis, systematic review, meta-study, concept analysis, rapid review, realist synthesis). The second begins as a broad general question that evolves and becomes more refined over the course of the synthesis (e.g., meta-ethnography, scoping review, meta-narrative, critical interpretive synthesis). In the third type, the synthesis begins with a phenomenon of interest and the question emerges in the analytic process (e.g., grounded formal theory). Lastly, there is no clear question, but rather a general review purpose (e.g., integrative review). Thus, the requirement for a well-defined question cuts across at least three of the synthesis types (e.g., quantitative, qualitative, and emerging).

4.1.4. Quality appraisal

This is a contested issue within and between the four synthesis categories. There are strong proponents of quality appraisal in the quantitative traditions of systematic review and meta-analysis based on the need for strong studies that will not jeopardize validity of the overall findings. Nonetheless, there is no consensus on pre-defined criteria; many scales exist that vary dramatically in composition. This has methodological implications for the credibility of findings [83] .

Specific methodologies from the conventional, qualitative, and emerging categories support quality appraisal but do so with caveats. In conventional integrative reviews appraisal is recommended, but depends on the sampling frame used in the study [18] . In meta-study, appraisal criteria are explicit but quality criteria are used in different ways depending on the specific requirements of the inquiry [54] . Among the emerging syntheses, meta-narrative review developers support appraisal of a study based on criteria from the research tradition of the primary study [67] , [84] – [85] . Realist synthesis similarly supports the use of high quality evidence, but appraisal checklists are viewed with scepticism and evidence is judged based on relevance to the research question and whether a credible inference may be drawn [69] . Like realist, critical interpretive syntheses do not judge quality using standardized appraisal instruments. They will exclude fatally flawed studies, but there is no consensus on what ‘fatally flawed’ means [49] , [71] . Appraisal is based on relevance to the inquiry, not rigor of the study.

There is no agreement on quality appraisal among qualitative meta-ethnographers with some supporting and others refuting the need for appraisal. [60] , [62] . Opponents of quality appraisal are found among authors of qualitative (grounded formal theory and concept analysis) and emerging syntheses (scoping and rapid reviews) because quality is not deemed relevant to the intention of the synthesis; the studies being reviewed are not effectiveness studies where quality is extremely important. These qualitative synthesis are often reviews of theoretical developments where the concept itself is what is important, or reviews that provide quotations from the raw data so readers can make their own judgements about the relevance and utility of the data. For example, in formal grounded theory, the purpose of theory generation and authenticity of data used to generate the theory is not as important as the conceptual category. Inaccuracies may be corrected in other ways, such as using the constant comparative method, which facilitates development of theoretical concepts that are repeatedly found in the data [86] – [87] . For pragmatic reasons, evidence is not assessed in rapid and scoping reviews, in part to produce a timely product. The issue of quality appraisal is unresolved across the terrain of research synthesis and we consider this further in our discussion.

4.2. Purpose

All research syntheses share a common purpose -- to summarize, synthesize, or integrate research findings from diverse studies. This helps readers stay abreast of the burgeoning literature in a field. Our discussion here is at the level of the four categories of synthesis. Beginning with conventional literature syntheses, the overall purpose is to attend to mature topics for the purpose of re-conceptualization or to new topics requiring preliminary conceptualization [14] . Such syntheses may be helpful to consider contradictory evidence, map shifting trends in the study of a phenomenon, and describe the emergence of research in diverse fields [14] . The purpose here is to set the stage for a study by identifying what has been done, gaps in the literature, important research questions, or to develop a conceptual framework to guide data collection and analysis.

The purpose of quantitative systematic reviews is to combine, aggregate, or integrate empirical research to be able to generalize from a group of studies and determine the limits of generalization [27] . The focus of quantitative systematic reviews has been primarily on aggregating the results of studies evaluating the effectiveness of interventions using experimental, quasi-experimental, and more recently, observational designs. Systematic reviews can be done with or without quantitative meta-analysis but a meta-analysis always takes place within the context of a systematic review. Researchers must consider the review's purpose and the nature of their data in undertaking a quantitative synthesis; this will assist in determining the approach.

The purpose of qualitative syntheses is broadly to synthesize complex health experiences, practices, or concepts arising in healthcare environments. There may be various purposes depending on the qualitative methodology. For example, in hermeneutic studies the aim may be holistic explanation or understanding of a phenomenon [42] , which is deepened by integrating the findings from multiple studies. In grounded formal theory, the aim is to produce a conceptual framework or theory expected to be applicable beyond the original study. Although not able to generalize from qualitative research in the statistical sense [88] , qualitative researchers usually do want to say something about the applicability of their synthesis to other settings or phenomena. This notion of ‘theoretical generalization’ has been referred to as ‘transferability’ [89] – [90] and is an important criterion of rigour in qualitative research. It applies equally to the products of a qualitative synthesis in which the synthesis of multiple studies on the same phenomenon strengthens the ability to draw transferable conclusions.

The overarching purpose of emerging syntheses is challenging the more traditional types of syntheses, in part by using data from both quantitative and qualitative studies with diverse designs for analysis. Beyond this, however, each emerging synthesis methodology has a unique purpose. In meta-narrative review, the purpose is to identify different research traditions in the area, synthesize a complex and diverse body of research. Critical interpretive synthesis shares this characteristic. Although a distinctive approach, critical interpretive synthesis utilizes a modification of the analytic strategies of meta-ethnography [61] (e.g., reciprocal translational analysis, refutational synthesis, and lines of argument synthesis) but goes beyond the use of these to bring a critical perspective to bear in challenging the normative or epistemological assumptions in the primary literature [72] – [73] . The unique purpose of a realist synthesis is to amalgamate complex empirical evidence and theoretical understandings within a diverse body of literature to uncover the operative mechanisms and contexts that affect the outcomes of social interventions. In a scoping review, the intention is to find key concepts, examine the range of research in an area, and identify gaps in the literature. The purpose of a rapid review is comparable to that of a scoping review, but done quickly to meet the time-sensitive information needs of policy makers.

4.3. Method

4.3.1. degree of systematization.

There are varying degrees of systematization across the categories of research synthesis. The most systematized are quantitative systematic reviews and meta-analyses. There are clear processes in each with judgments to be made at each step, although there are no agreed upon guidelines for this. The process is inherently subjective despite attempts to develop objective and systematic processes [91] – [92] . Mullen and Ramirez [27] suggest that there is often a false sense of rigour implied by the terms ‘systematic review’ and ‘meta-analysis’ because of their clearly defined procedures.

In comparison with some types of qualitative synthesis, concept analysis is quite procedural. Qualitative meta-synthesis also has defined procedures and is systematic, yet perhaps less so than concept analysis. Qualitative meta-synthesis starts in an unsystematic way but becomes more systematic as it unfolds. Procedures and frameworks exist for some of the emerging types of synthesis [e.g., [50] , [63] , [71] , [93] ] but are not linear, have considerable flexibility, and are often messy with emergent processes [85] . Conventional literature reviews tend not to be as systematic as the other three types. In fact, the lack of systematization in conventional literature synthesis was the reason for the development of more systematic quantitative [17] , [20] and qualitative [45] – [46] , [61] approaches. Some authors in the field [18] have clarified processes for integrative reviews making them more systematic and rigorous, but most conventional syntheses remain relatively unsystematic in comparison with other types.

4.3.2. Complexity of the process

Some synthesis processes are considerably more complex than others. Methodologies with clearly defined steps are arguably less complex than the more flexible and emergent ones. We know that any study encounters challenges and it is rare that a pre-determined research protocol can be followed exactly as intended. Not even the rigorous methods associated with Cochrane [81] systematic reviews and meta-analyses are always implemented exactly as intended. Even when dealing with numbers rather than words, interpretation is always part of the process. Our collective experience suggests that new methodologies (e.g., meta-narrative synthesis and realist synthesis) that integrate different data types and methods are more complex than conventional reviews or the rapid and scoping reviews.

4.4. Product

The products of research syntheses usually take three distinct formats (see Table 1 and Additional File 1 for further details). The first representation is in tables, charts, graphical displays, diagrams and maps as seen in integrative, scoping and rapid reviews, meta-analyses, and critical interpretive syntheses. The second type of synthesis product is the use of mathematical scores. Summary statements of effectiveness are mathematically displayed in meta-analyses (as an effect size), systematic reviews, and rapid reviews (statistical significance).

The third synthesis product may be a theory or theoretical framework. A mid-range theory can be produced from formal grounded theory, meta-study, meta-ethnography, and realist synthesis. Theoretical/conceptual frameworks or conceptual maps may be created in meta-narrative and critical interpretive syntheses, and integrative reviews. Concepts for use within theories are produced in concept analysis. While these three product types span the categories of research synthesis, narrative description and summary is used to present the products resulting from all methodologies.

4.5. Consideration of context

There are diverse ways that context is considered in the four broad categories of synthesis. Context may be considered to the extent that it features within primary studies for the purpose of the review. Context may also be understood as an integral aspect of both the phenomenon under study and the synthesis methodology (e.g., realist synthesis). Quantitative systematic reviews and meta-analyses have typically been conducted on studies using experimental and quasi-experimental designs and more recently observational studies, which control for contextual features to allow for understanding of the ‘true’ effect of the intervention [94] .

More recently, systematic reviews have included covariates or mediating variables (i.e., contextual factors) to help explain variability in the results across studies [27] . Context, however, is usually handled in the narrative discussion of findings rather than in the synthesis itself. This lack of attention to context has been one criticism leveled against systematic reviews and meta-analyses, which restrict the types of research designs that are considered [e.g., [95] ].

When conventional literature reviews incorporate studies that deal with context, there is a place for considering contextual influences on the intervention or phenomenon. Reviews of quantitative experimental studies tend to be devoid of contextual considerations since the original studies are similarly devoid, but context might figure prominently in a literature review that incorporates both quantitative and qualitative studies.

Qualitative syntheses have been conducted on the contextual features of a particular phenomenon [33] . Paterson et al. [54] advise researchers to attend to how context may have influenced the findings of particular primary studies. In qualitative analysis, contextual features may form categories by which the data can be compared and contrasted to facilitate interpretation. Because qualitative research is often conducted to understand a phenomenon as a whole, context may be a focus, although this varies with the qualitative methodology. At the same time, the findings in a qualitative synthesis are abstracted from the original reports and taken to a higher level of conceptualization, thus removing them from the original context.

Meta-narrative synthesis [67] , [84] , because it draws on diverse research traditions and methodologies, may incorporate context into the analysis and findings. There is not, however, an explicit step in the process that directs the analyst to consider context. Generally, the research question guiding the synthesis is an important factor in whether context will be a focus.

More recent iterations of concept analysis [47] , [96] – [97] explicitly consider context reflecting the assumption that a concept's meaning is determined by its context. Morse [47] points out, however, that Wilson's [98] approach to concept analysis, and those based on Wilson [e.g., [45] ], identify attributes that are devoid of context, while Rodgers' [96] , [99] evolutionary method considers context (e.g., antecedents, consequences, and relationships to other concepts) in concept development.

Realist synthesis [69] considers context as integral to the study. It draws on a critical realist logic of inquiry grounded in the work of Bhaskar [100] , who argues that empirical co-occurrence of events is insufficient for inferring causation. One must identify generative mechanisms whose properties are causal and, depending on the situation, may nor may not be activated [94] . Context interacts with program/intervention elements and thus cannot be differentiated from the phenomenon [69] . This approach synthesizes evidence on generative mechanisms and analyzes contextual features that activate them; the result feeds back into the context. The focus is on what works, for whom, under what conditions, why and how [68] .

4.6. Underlying Philosophical and Theoretical Assumptions

When we began our review, we ‘assumed’ that the assumptions underlying synthesis methodologies would be a distinguishing characteristic of synthesis types, and that we could compare the various types on their assumptions, explicit or implicit. We found, however, that many authors did not explicate the underlying assumptions of their methodologies, and it was difficult to infer them. Kirkevold [101] has argued that integrative reviews need to be carried out from an explicit philosophical or theoretical perspective. We argue this should be true for all types of synthesis.

Authors of some emerging synthesis approaches have been very explicit about their assumptions and philosophical underpinnings. An implicit assumption of most emerging synthesis methodologies is that quantitative systematic reviews and meta-analyses have limited utility in some fields [e.g., in public health – [13] , [102] ] and for some kinds of review questions like those about feasibility and appropriateness versus effectiveness [103] – [104] . They also assume that ontologically and epistemologically, both kinds of data can be combined. This is a significant debate in the literature because it is about the commensurability of overarching paradigms [105] but this is beyond the scope of this review.

Realist synthesis is philosophically grounded in critical realism or, as noted above, a realist logic of inquiry [93] , [99] , [106] – [107] . Key assumptions regarding the nature of interventions that inform critical realism have been described above in the section on context. See Pawson et al. [106] for more information on critical realism, the philosophical basis of realist synthesis.

Meta-narrative synthesis is explicitly rooted in a constructivist philosophy of science [108] in which knowledge is socially constructed rather than discovered, and what we take to be ‘truth’ is a matter of perspective. Reality has a pluralistic and plastic character, and there is no pre-existing ‘real world’ independent of human construction and language [109] . See Greenhalgh et al. [67] , [85] and Greenhalgh & Wong [97] for more discussion of the constructivist basis of meta-narrative synthesis.

In the case of purely quantitative or qualitative syntheses, it may be an easier matter to uncover unstated assumptions because they are likely to be shared with those of the primary studies in the genre. For example, grounded formal theory shares the philosophical and theoretical underpinnings of grounded theory, rooted in the theoretical perspective of symbolic interactionism [110] – [111] and the philosophy of pragmatism [87] , [112] – [114] .

As with meta-narrative synthesis, meta-study developers identify constructivism as their interpretive philosophical foundation [54] , [88] . Epistemologically, constructivism focuses on how people construct and re-construct knowledge about a specific phenomenon, and has three main assumptions: (1) reality is seen as multiple, at times even incompatible with the phenomenon under consideration; (2) just as primary researchers construct interpretations from participants' data, meta-study researchers also construct understandings about the primary researchers' original findings. Thus, meta-synthesis is a construction of a construction, or a meta-construction; and (3) all constructions are shaped by the historical, social and ideological context in which they originated [54] . The key message here is that reports of any synthesis would benefit from an explicit identification of the underlying philosophical perspectives to facilitate a better understanding of the results, how they were derived, and how they are being interpreted.

4.7. Unit of Analysis

The unit of analysis for each category of review is generally distinct. For the emerging synthesis approaches, the unit of analysis is specific to the intention. In meta-narrative synthesis it is the storyline in diverse research traditions; in rapid review or scoping review, it depends on the focus but could be a concept; and in realist synthesis, it is the theories rather than programs that are the units of analysis. The elements of theory that are important in the analysis are mechanisms of action, the context, and the outcome [107] .

For qualitative synthesis, the units of analysis are generally themes, concepts or theories, although in meta-study, the units of analysis can be research findings (“meta-data-analysis”), research methods (“meta-method”) or philosophical/theoretical perspectives (“meta-theory”) [54] . In quantitative synthesis, the units of analysis range from specific statistics for systematic reviews to effect size of the intervention for meta-analysis. More recently, some systematic reviews focus on theories [115] – [116] , therefore it depends on the research question. Similarly, within conventional literature synthesis the units of analysis also depend on the research purpose, focus and question as well as on the type of research methods incorporated into the review. What is important in all research syntheses, however, is that the unit of analysis needs to be made explicit. Unfortunately, this is not always the case.

4.8. Strengths and Limitations

In this section, we discuss the overarching strengths and limitations of synthesis methodologies as a whole and then highlight strengths and weaknesses across each of our four categories of synthesis.

4.8.1. Strengths of Research Syntheses in General

With the vast proliferation of research reports and the increased ease of retrieval, research synthesis has become more accessible providing a way of looking broadly at the current state of research. The availability of syntheses helps researchers, practitioners, and policy makers keep up with the burgeoning literature in their fields without which evidence-informed policy or practice would be difficult. Syntheses explain variation and difference in the data helping us identify the relevance for our own situations; they identify gaps in the literature leading to new research questions and study designs. They help us to know when to replicate a study and when to avoid excessively duplicating research. Syntheses can inform policy and practice in a way that well-designed single studies cannot; they provide building blocks for theory that helps us to understand and explain our phenomena of interest.

4.8.2. Limitations of Research Syntheses in General

The process of selecting, combining, integrating, and synthesizing across diverse study designs and data types can be complex and potentially rife with bias, even with those methodologies that have clearly defined steps. Just because a rigorous and standardized approach has been used does not mean that implicit judgements will not influence the interpretations and choices made at different stages.

In all types of synthesis, the quantity of data can be considerable, requiring difficult decisions about scope, which may affect relevance. The quantity of available data also has implications for the size of the research team. Few reviews these days can be done independently, in particular because decisions about inclusion and exclusion may require the involvement of more than one person to ensure reliability.

For all types of synthesis, it is likely that in areas with large, amorphous, and diverse bodies of literature, even the most sophisticated search strategies will not turn up all the relevant and important texts. This may be more important in some synthesis methodologies than in others, but the omission of key documents can influence the results of all syntheses. This issue can be addressed, at least in part, by including a library scientist on the research team as required by some funding agencies. Even then, it is possible to miss key texts. In this review, for example, because none of us are trained in or conduct meta-analyses, we were not even aware that we had missed some new developments in this field such as meta-regression [117] – [118] , network meta-analysis [119] – [121] , and the use of individual patient data in meta-analyses [122] – [123] .

One limitation of systematic reviews and meta-analyses is that they rapidly go out of date. We thought this might be true for all types of synthesis, although we wondered if those that produce theory might not be somewhat more enduring. We have not answered this question but it is open for debate. For all types of synthesis, the analytic skills and the time required are considerable so it is clear that training is important before embarking on a review, and some types of review may not be appropriate for students or busy practitioners.

Finally, the quality of reporting in primary studies of all genres is variable so it is sometimes difficult to identify aspects of the study essential for the synthesis, or to determine whether the study meets quality criteria. There may be flaws in the original study, or journal page limitations may necessitate omitting important details. Reporting standards have been developed for some types of reviews (e.g., systematic review, meta-analysis, meta-narrative synthesis, realist synthesis); but there are no agreed upon standards for qualitative reviews. This is an important area for development in advancing the science of research synthesis.

4.8.3. Strengths and Limitations of the Four Synthesis Types

The conventional literature review and now the increasingly common integrative review remain important and accessible approaches for students, practitioners, and experienced researchers who want to summarize literature in an area but do not have the expertise to use one of the more complex methodologies. Carefully executed, such reviews are very useful for synthesizing literature in preparation for research grants and practice projects. They can determine the state of knowledge in an area and identify important gaps in the literature to provide a clear rationale or theoretical framework for a study [14] , [18] . There is a demand, however, for more rigour, with more attention to developing comprehensive search strategies and more systematic approaches to combining, integrating, and synthesizing the findings.

Generally, conventional reviews include diverse study designs and data types that facilitate comprehensiveness, which may be a strength on the one hand, but can also present challenges on the other. The complexity inherent in combining results from studies with diverse methodologies can result in bias and inaccuracies. The absence of clear guidelines about how to synthesize across diverse study types and data [18] has been a challenge for novice reviewers.

Quantitative systematic reviews and meta-analyses have been important in launching the field of evidence-based healthcare. They provide a systematic, orderly and auditable process for conducting a review and drawing conclusions [25] . They are arguably the most powerful approaches to understanding the effectiveness of healthcare interventions, especially when intervention studies on the same topic show very different results. When areas of research are dogged by controversy [25] or when study results go against strongly held beliefs, such approaches can reduce the uncertainty and bring strong evidence to bear on the controversy.

Despite their strengths, they also have limitations. Systematic reviews and meta-analyses do not provide a way of including complex literature comprising various types of evidence including qualitative studies, theoretical work, and epidemiological studies. Only certain types of design are considered and qualitative data are used in a limited way. This exclusion limits what can be learned in a topic area.

Meta-analyses are often not possible because of wide variability in study design, population, and interventions so they may have a narrow range of utility. New developments in meta-analysis, however, can be used to address some of these limitations. Network meta-analysis is used to explore relative efficacy of multiple interventions, even those that have never been compared in more conventional pairwise meta-analyses [121] , allowing for improved clinical decision making [120] . The limitation is that network meta-analysis has only been used in medical/clinical applications [119] and not in public health. It has not yet been widely accepted and many methodological challenges remain [120] – [121] . Meta-regression is another development that combines meta-analytic and linear regression principles to address the fact that heterogeneity of results may compromise a meta-analysis [117] – [118] . The disadvantage is that many clinicians are unfamiliar with it and may incorrectly interpret results [117] .

Some have accused meta-analysis of combining apples and oranges [124] raising questions in the field about their meaningfulness [25] , [28] . More recently, the use of individual rather than aggregate data has been useful in facilitating greater comparability among studies [122] . In fact, Tomas et al. [123] argue that meta-analysis using individual data is now the gold standard although access to the raw data from other studies may be a challenge to obtain.

The usefulness of systematic reviews in synthesizing complex health and social interventions has also been challenged [102] . It is often difficult to synthesize their findings because such studies are “epistemologically diverse and methodologically complex” [ [69] , p.21]. Rigid inclusion/exclusion criteria may allow only experimental or quasi-experimental designs into consideration resulting in lost information that may well be useful to policy makers for tailoring an intervention to the context or understanding its acceptance by recipients.

Qualitative syntheses may be the type of review most fraught with controversy and challenge, while also bringing distinct strengths to the enterprise. Although these methodologies provide a comprehensive and systematic review approach, they do not generally provide definitive statements about intervention effectiveness. They do, however, address important questions about the development of theoretical concepts, patient experiences, acceptability of interventions, and an understanding about why interventions might work.

Most qualitative syntheses aim to produce a theoretically generalizable mid-range theory that explains variation across studies. This makes them more useful than single primary studies, which may not be applicable beyond the immediate setting or population. All provide a contextual richness that enhances relevance and understanding. Another benefit of some types of qualitative synthesis (e.g., grounded formal theory) is that the concept of saturation provides a sound rationale for limiting the number of texts to be included thus making reviews potentially more manageable. This contrasts with the requirements of systematic reviews and meta-analyses that require an exhaustive search.

Qualitative researchers debate about whether the findings of ontologically and epistemological diverse qualitative studies can actually be combined or synthesized [125] because methodological diversity raises many challenges for synthesizing findings. The products of different types of qualitative syntheses range from theory and conceptual frameworks, to themes and rich descriptive narratives. Can one combine the findings from a phenomenological study with the theory produced in a grounded theory study? Many argue yes, but many also argue no.

Emerging synthesis methodologies were developed to address some limitations inherent in other types of synthesis but also have their own issues. Because each type is so unique, it is difficult to identify overarching strengths of the entire category. An important strength, however, is that these newer forms of synthesis provide a systematic and rigorous approach to synthesizing a diverse literature base in a topic area that includes a range of data types such as: both quantitative and qualitative studies, theoretical work, case studies, evaluations, epidemiological studies, trials, and policy documents. More than conventional literature reviews and systematic reviews, these approaches provide explicit guidance on analytic methods for integrating different types of data. The assumption is that all forms of data have something to contribute to knowledge and theory in a topic area. All have a defined but flexible process in recognition that the methods may need to shift as knowledge develops through the process.

Many emerging synthesis types are helpful to policy makers and practitioners because they are usually involved as team members in the process to define the research questions, and interpret and disseminate the findings. In fact, engagement of stakeholders is built into the procedures of the methods. This is true for rapid reviews, meta-narrative syntheses, and realist syntheses. It is less likely to be the case for critical interpretive syntheses.

Another strength of some approaches (realist and meta-narrative syntheses) is that quality and publication standards have been developed to guide researchers, reviewers, and funders in judging the quality of the products [108] , [126] – [127] . Training materials and online communities of practice have also been developed to guide users of realist and meta-narrative review methods [107] , [128] . A unique strength of critical interpretive synthesis is that it takes a critical perspective on the process that may help reconceptualize the data in a way not considered by the primary researchers [72] .

There are also challenges of these new approaches. The methods are new and there may be few published applications by researchers other than the developers of the methods, so new users often struggle with the application. The newness of the approaches means that there may not be mentors available to guide those unfamiliar with the methods. This is changing, however, and the number of applications in the literature is growing with publications by new users helping to develop the science of synthesis [e.g., [129] ]. However, the evolving nature of the approaches and their developmental stage present challenges for novice researchers.

4.9. When to Use Each Approach

Choosing an appropriate approach to synthesis will depend on the question you are asking, the purpose of the review, and the outcome or product you want to achieve. In Additional File 1 , we discuss each of these to provide guidance to readers on making a choice about review type. If researchers want to know whether a particular type of intervention is effective in achieving its intended outcomes, then they might choose a quantitative systemic review with or without meta-analysis, possibly buttressed with qualitative studies to provide depth and explanation of the results. Alternately, if the concern is about whether an intervention is effective with different populations under diverse conditions in varying contexts, then a realist synthesis might be the most appropriate.

If researchers' concern is to develop theory, they might consider qualitative syntheses or some of the emerging syntheses that produce theory (e.g., critical interpretive synthesis, realist review, grounded formal theory, qualitative meta-synthesis). If the aim is to track the development and evolution of concepts, theories or ideas, or to determine how an issue or question is addressed across diverse research traditions, then meta-narrative synthesis would be most appropriate.

When the purpose is to review the literature in advance of undertaking a new project, particularly by graduate students, then perhaps an integrative review would be appropriate. Such efforts contribute towards the expansion of theory, identify gaps in the research, establish the rationale for studying particular phenomena, and provide a framework for interpreting results in ways that might be useful for influencing policy and practice.

For researchers keen to bring new insights, interpretations, and critical re-conceptualizations to a body of research, then qualitative or critical interpretive syntheses will provide an inductive product that may offer new understandings or challenges to the status quo. These can inform future theory development, or provide guidance for policy and practice.

5. Discussion

What is the current state of science regarding research synthesis? Public health, health care, and social science researchers or clinicians have previously used all four categories of research synthesis, and all offer a suitable array of approaches for inquiries. New developments in systematic reviews and meta-analysis are providing ways of addressing methodological challenges [117] – [123] . There has also been significant advancement in emerging synthesis methodologies and they are quickly gaining popularity. Qualitative meta-synthesis is still evolving, particularly given how new it is within the terrain of research synthesis. In the midst of this evolution, outstanding issues persist such as grappling with: the quantity of data, quality appraisal, and integration with knowledge translation. These topics have not been thoroughly addressed and need further debate.

5.1. Quantity of Data

We raise the question of whether it is possible or desirable to find all available studies for a synthesis that has this requirement (e.g., meta-analysis, systematic review, scoping, meta-narrative synthesis [25] , [27] , [63] , [67] , [84] – [85] ). Is the synthesis of all available studies a realistic goal in light of the burgeoning literature? And how can this be sustained in the future, particularly as the emerging methodologies continue to develop and as the internet facilitates endless access? There has been surprisingly little discussion on this topic and the answers will have far-reaching implications for searching, sampling, and team formation.

Researchers and graduate students can no longer rely on their own independent literature search. They will likely need to ask librarians for assistance as they navigate multiple sources of literature and learn new search strategies. Although teams now collaborate with library scientists, syntheses are limited in that researchers must make decisions on the boundaries of the review, in turn influencing the study's significance. The size of a team may also be pragmatically determined to manage the search, extraction, and synthesis of the burgeoning data. There is no single answer to our question about the possibility or necessity of finding all available articles for a review. Multiple strategies that are situation specific are likely to be needed.

5.2. Quality Appraisal

While the issue of quality appraisal has received much attention in the synthesis literature, scholars are far from resolution. There may be no agreement about appraisal criteria in a given tradition. For example, the debate rages over the appropriateness of quality appraisal in qualitative synthesis where there are over 100 different sets of criteria and many do not overlap [49] . These differences may reflect disciplinary and methodological orientations, but diverse quality appraisal criteria may privilege particular types of research [49] . The decision to appraise is often grounded in ontological and epistemological assumptions. Nonetheless, diversity within and between categories of synthesis is likely to continue unless debate on the topic of quality appraisal continues and evolves toward consensus.

5.3. Integration with Knowledge Translation

If research syntheses are to make a difference to practice and ultimately to improve health outcomes, then we need to do a better job of knowledge translation. In the Canadian Institutes of Health Research (CIHR) definition of knowledge translation (KT), research or knowledge synthesis is an integral component [130] . Yet, with few exceptions [131] – [132] , very little of the research synthesis literature even mentions the relationship of synthesis to KT nor does it discuss strategies to facilitate the integration of synthesis findings into policy and practice. The exception is in the emerging synthesis methodologies, some of which (e.g., realist and meta-narrative syntheses, scoping reviews) explicitly involve stakeholders or knowledge users. The argument is that engaging them in this way increases the likelihood that the knowledge generated will be translated into policy and practice. We suggest that a more explicit engagement with knowledge users in all types of synthesis would benefit the uptake of the research findings.

Research synthesis neither makes research more applicable to practice nor ensures implementation. Focus must now turn seriously towards translation of synthesis findings into knowledge products that are useful for health care practitioners in multiple areas of practice and develop appropriate strategies to facilitate their use. The burgeoning field of knowledge translation has, to some extent, taken up this challenge; however, the research-practice gap continues to plague us [133] – [134] . It is a particular problem for qualitative syntheses [131] . Although such syntheses have an important place in evidence-informed practice, little effort has gone into the challenge of translating the findings into useful products to guide practice [131] .

5.4. Limitations

Our study took longer than would normally be expected for an integrative review. Each of us were primarily involved in our own dissertations or teaching/research positions, and so this study was conducted ‘off the sides of our desks.’ A limitation was that we searched the literature over the course of 4 years (from 2008–2012), necessitating multiple search updates. Further, we did not do a comprehensive search of the literature after 2012, thus the more recent synthesis literature was not systematically explored. We did, however, perform limited database searches from 2012–2015 to keep abreast of the latest methodological developments. Although we missed some new approaches to meta-analysis in our search, we did not find any new features of the synthesis methodologies covered in our review that would change the analysis or findings of this article. Lastly, we struggled with the labels used for the broad categories of research synthesis methodology because of our hesitancy to reinforce the divide between quantitative and qualitative approaches. However, it was very difficult to find alternative language that represented the types of data used in these methodologies. Despite our hesitancy in creating such an obvious divide, we were left with the challenge of trying to find a way of characterizing these broad types of syntheses.

6. Conclusion

Our findings offer methodological clarity for those wishing to learn about the broad terrain of research synthesis. We believe that our review makes transparent the issues and considerations in choosing from among the four broad categories of research synthesis. In summary, research synthesis has taken its place as a form of research in its own right. The methodological terrain has deep historical roots reaching back over the past 200 years, yet research synthesis remains relatively new to public health, health care, and social sciences in general. This is rapidly changing. New developments in systematic reviews and meta-analysis, and the emergence of new synthesis methodologies provide a vast array of options to review the literature for diverse purposes. New approaches to research synthesis and new analytic methods within existing approaches provide a much broader range of review alternatives for public health, health care, and social science students and researchers.

Acknowledgments

KSM is an assistant professor in the Faculty of Nursing at the University of Alberta. Her work on this article was largely conducted as a Postdoctoral Fellow, funded by KRESCENT (Kidney Research Scientist Core Education and National Training Program, reference #KRES110011R1) and the Faculty of Nursing at the University of Alberta.

MM's work on this study over the period of 2008-2014 was supported by a Canadian Institutes of Health Research Applied Public Health Research Chair Award (grant #92365).

We thank Rachel Spanier who provided support with reference formatting.

List of Abbreviations (in Additional File 1 )

Conflict of interest: The authors declare that they have no conflicts of interest in this article.

Authors' contributions: KSM co-designed the study, collected data, analyzed the data, drafted/revised the manuscript, and managed the project.

MP contributed to searching the literature, developing the analytic framework, and extracting data for the Additional File.

JB contributed to searching the literature, developing the analytic framework, and extracting data for the Additional File.

WN contributed to searching the literature, developing the analytic framework, and extracting data for the Additional File.

All authors read and approved the final manuscript.

Additional Files: Additional File 1 – Selected Types of Research Synthesis

This Additional File is our dataset created to organize, analyze and critique the literature that we synthesized in our integrative review. Our results were created based on analysis of this Additional File.

A Guide to Evidence Synthesis: What is Evidence Synthesis?

• Meet Our Team
• Our Published Reviews and Protocols
• What is Evidence Synthesis?
• Types of Evidence Synthesis
• Evidence Synthesis Across Disciplines
• Finding and Appraising Existing Systematic Reviews
• 0. Develop a Protocol
• 1. Draft your Research Question
• 2. Select Databases
• 3. Select Grey Literature Sources
• 4. Write a Search Strategy
• 5. Register a Protocol
• 6. Translate Search Strategies
• 7. Citation Management
• 8. Article Screening
• 9. Risk of Bias Assessment
• 10. Data Extraction
• 11. Synthesize, Map, or Describe the Results
• Evidence Synthesis Institute for Librarians
• Open Access Evidence Synthesis Resources

What are Evidence Syntheses?

What are evidence syntheses.

According to the Royal Society, 'evidence synthesis' refers to the process of bringing together information from a range of sources and disciplines to inform debates and decisions on specific issues. They generally include a methodical and comprehensive literature synthesis focused on a well-formulated research question.  Their aim is to identify and synthesize all  of the scholarly research on a particular topic, including both published and unpublished studies. Evidence syntheses are conducted in an unbiased, reproducible way to provide evidence for practice and policy-making, as well as to identify gaps in the research. Evidence syntheses may also include a meta-analysis, a more quantitative process of synthesizing and visualizing data retrieved from various studies. 

Evidence syntheses are much more time-intensive than traditional literature reviews and require a multi-person research team. See this PredicTER tool to get a sense of a systematic review timeline (one type of evidence synthesis). Before embarking on an evidence synthesis, it's important to clearly identify your reasons for conducting one. For a list of types of evidence synthesis projects, see the next tab.

How Does a Traditional Literature Review Differ From an Evidence Synthesis?

How does a systematic review differ from a traditional literature review.

One commonly used form of evidence synthesis is a systematic review.  This table compares a traditional literature review with a systematic review.

Video: Reproducibility and transparent methods (Video 3:25)

Reporting Standards

There are some reporting standards for evidence syntheses. These can serve as guidelines for protocol and manuscript preparation and journals may require that these standards are followed for the review type that is being employed (e.g. systematic review, scoping review, etc). ​

• PRISMA checklist Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) is an evidence-based minimum set of items for reporting in systematic reviews and meta-analyses.
• PRISMA-P Standards An updated version of the original PRISMA standards for protocol development.
• PRISMA - ScR Reporting guidelines for scoping reviews and evidence maps
• PRISMA-IPD Standards Extension of the original PRISMA standards for systematic reviews and meta-analyses of individual participant data.
• EQUATOR Network The EQUATOR (Enhancing the QUAlity and Transparency Of health Research) Network is an international initiative that seeks to improve the reliability and value of published health research literature by promoting transparent and accurate reporting and wider use of robust reporting guidelines. They provide a list of various standards for reporting in systematic reviews.

Video: Guidelines and reporting standards

PRISMA Flow Diagram

The  PRISMA  flow diagram depicts the flow of information through the different phases of an evidence synthesis. It maps the search (number of records identified), screening (number of records included and excluded), and selection (reasons for exclusion).  Many evidence syntheses include a PRISMA flow diagram in the published manuscript.

See below for resources to help you generate your own PRISMA flow diagram.

• PRISMA Flow Diagram Tool
• PRISMA Flow Diagram Word Template
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Get Organized

• Lit Review Prep Use this template to help you evaluate your sources, create article summaries for an annotated bibliography, and a synthesis matrix for your lit review outline.

Synthesize your Information

Synthesize: combine separate elements to form a whole.

Synthesis Matrix

A synthesis matrix helps you record the main points of each source and document how sources relate to each other.

After summarizing and evaluating your sources, arrange them in a matrix or use a citation manager to help you see how they relate to each other and apply to each of your themes or variables.  

By arranging your sources by theme or variable, you can see how your sources relate to each other, and can start thinking about how you weave them together to create a narrative.

• Step-by-Step Approach
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• Last Updated: Sep 26, 2023 10:25 AM
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• How it works

How to Synthesise Sources – Steps and Examples

Published by Olive Robin at October 17th, 2023 , Revised On October 17, 2023

The ability to effectively incorporate multiple sources into one’s work is not just a skill, but a necessity. Whether we are talking about research papers, articles, or even simple blog posts, synthesising sources can elevate our content to a more nuanced, comprehensive, and insightful level. But what does it truly mean to synthesise sources, and how does it differ from the commonly understood techniques of summarising and paraphrasing?

Importance of Synthesising Sources in Research and Writing

When finding sources , it is imperative to distinguish between various available information types. Secondary sources , for example, provide interpretations and analyses based on primary sources. Synthesising goes beyond the mere gathering of information. It involves the complex task of interweaving multiple sources to generate a broader and richer perspective. When we synthesise, we are not just collecting; we are connecting. 

By merging various viewpoints and data, we provide our readers with a well-rounded understanding of the topic. This approach ensures that our work is grounded in credible sources  while also adding unique insights.

Summarising, Paraphrasing, and Synthesising

At first glance, these three techniques might seem similar, but they serve distinctly different purposes:

Summarising

Synthesis is the art of blending multiple sources to create a unified narrative or argument. It is essential to use signal phrases to introduce these sources naturally, helping the reader follow the information flow. Block quotes can also be used for direct quotations, especially if they’re longer.

Paraphrasing

Here, we restate the original content using different words. While the wording changes, the essence and core meaning remain intact. This method is useful for clarifying complex ideas or for tailoring content to a specific audience.

Synthesising

Synthesis is the art of blending multiple sources to create a unified narrative or argument. It is not about echoing what others have said; it is about drawing connections, identifying patterns, and building a cohesive piece that holds its own merit.

What is Synthesising Sources?

Synthesising sources is a method used in research and writing wherein the author combines, interprets, and analyses information from various sources to generate a unified perspective, narrative, or argument. It involves a process that we can call source evaluation . It is an intricate process that goes beyond simply collecting data or quoting authors. Instead, it involves evaluating, integrating, and constructing a new narrative based on a collective understanding of all the sources under consideration.

Imagine a quilt where each piece of fabric represents a different source. Synthesising would be the act of sewing these individual pieces together in such a way that they form a beautiful, cohesive blanket. Each piece retains its uniqueness but contributes to the larger design and purpose of the quilt.

Objective of Synthesising

• Synthesising allows writers to delve deeper into topics by using a multitude of perspectives. This offers a more robust and comprehensive view than any single source could provide.
• By integrating diverse sources, authors can identify trends, consistencies, or discrepancies within a field or topic. This can lead to new insights or highlight areas needing further exploration.
• By interlinking sources, writers can add layers of complexity to their arguments, making their content more engaging and thought-provoking.
• While the sources themselves might not be new, the way in which they are combined and interpreted can lead to fresh conclusions and unique standpoints.
• Relying on a single source or a few like-minded ones can inadvertently introduce biases. Synthesising encourages the consideration of diverse viewpoints, ensuring a more balanced representation of the topic.

Why is Synthesis Important?

The art of synthesis, while a nuanced aspect of research and writing, holds unparalleled significance in constructing meaningful, in-depth content. Here is a detailed exploration of why synthesis is pivotal:

Enhancing Comprehension and Knowledge Depth

• Depth Over Breadth: While a vast amount of information exists on nearly any topic, true understanding isn’t about skimming the surface. Synthesising allows you to dive deeper, connecting various pieces of information and seeing the bigger picture.
• Clarifying Complexity: Topics, especially those in research, can be multifaceted. By merging multiple sources, we can simplify and explain intricate subjects more effectively.
• Reinforcing Concepts: By revisiting a concept from various sources and angles, the repetition, in a way, strengthens our grasp on the subject. It’s like studying from multiple textbooks; the overlap in content solidifies understanding.

Avoiding Plagiarism

• Original Thought Generation: While synthesising, you are compelled to merge ideas, compare viewpoints, and draw unique conclusions. This process naturally leads to producing original content rather than merely reproducing what one source says.
• Skilful Integration: A well-synthesised piece does not heavily rely on long, verbatim quotes. Instead, it seamlessly integrates information from various sources, duly cited, minimising the chances of unintentional plagiarism.
• Reflecting Authentic Engagement: When you synthesise, it showcases your genuine engagement with the material. It’s evident that you have not just copied content but have wrestled with the information, pondered upon it, and made it your own.

Developing a Holistic Perspective on a Topic

• Seeing the Full Spectrum: Single sources can offer a limited or biased viewpoint. Synthesis, by its nature, compels you to consult multiple sources, allowing for a more balanced and comprehensive view.
• Connecting the Dots: Life, society, and most academic subjects are interconnected. Synthesis helps recognise patterns, draw parallels, and understand how various elements interplay in the grand scheme of things.
• Elevating Critical Thinking: The act of synthesis hones your critical thinking skills. You’re constantly evaluating the validity of sources, comparing arguments, and discerning the weight of different perspectives. This makes your current work stronger and sharpens your intellect for future projects.

Steps of Synthesizing a Source

Here is a step-by-step guide on how to synthesise sources. 

Step 1: Read and Understand

Before you can synthesise sources effectively, you must first understand them individually. A strong synthesis is built upon a clear understanding of each source’s content, context, and nuances.

Tips To Ensure Comprehension

• Annotations: Make notes in the margins as you read, highlighting key points and ideas.
• Summarisation: After reading a section or an article, write a brief summary in your own words.
• Discussion: Talk about the content with peers or mentors. This can help clarify any confusion and deepen your understanding.
• Questioning: Constantly ask questions as you read. If something is unclear, revisit the content or consult supplementary materials.

Step 2: Identify Common Themes

Sources will often touch upon similar themes, even if they approach them differently. Recognising these themes can act as a foundation for synthesis.

• Mind Mapping: Visualise the interconnectedness of topics and subtopics.
• Lists: Create lists of similar ideas or arguments from different sources.
• Highlighting: Use colour codes to highlight recurring themes across different documents.

Step 3: Analyse and Compare

Different sources might have diverging opinions or findings. Recognising these differences is crucial to produce a balanced synthesis.

• Side-by-Side Analysis: Put the information from various sources next to each other to see how they align or diverge.
• Critical Evaluation: Ask yourself why sources might have different perspectives. Consider the methodology, context, or biases that could contribute.

Determining the Relevance of Each Source

Not all sources will hold equal weight or relevance in your synthesis.

• Criteria Checklist: Establish criteria for relevance (e.g., publication date, author credentials) and evaluate each source against this.
• Priority Setting: Decide which sources offer primary insights and which offer supplementary information.

Step 4: Organise Information

A clear structure is essential to guide your readers through the synthesised narrative.

• Outlines: Create a traditional outline that sequences your main points and supports them with subpoints from your sources.
• Flowcharts: For more complex topics, flowcharts can visually demonstrate the progression of ideas and their interconnections.

Step 5: Craft Your Narrative

This step involves the actual writing, where you combine the insights, evidence, and analysis into a singular narrative.

• Transitional Phrasing: Use transitions to move between ideas and sources smoothly.
• Voice Consistency: Even though you integrate multiple sources, ensure that the narrative maintains a consistent voice and tone.

Step 6: Cite Appropriately

Always credit original authors and sources to maintain integrity in your work and avoid plagiarism. Knowing how to cite sources is crucial in this process.

• In-text Citations: Whenever you refer to, paraphrase, or quote a source, provide a citation.

Different Citation Styles and Choosing The Right One

There are multiple citation styles (e.g., APA, MLA, Chicago), and your choice will often depend on your discipline or the preference of your institution or publication.

• Guideline Review: Familiarise yourself with the preferred citation style’s guidelines.

Citation Tools: Consider using tools like Zotero, Mendeley, or EndNote to help streamline and manage your citations.

The research done by our experts have:

• Precision and Clarity
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Examples of Source Synthesis

Let’s explore some examples of synthesising sources. 

Example 1: Synthesising sources on climate change

Scenario: You have sources that discuss the causes of climate change. Some sources argue for anthropogenic (human-caused) factors, while others emphasise natural cycles.

Synthesis Approach

• Begin with an overview of climate change, its impacts, and its significance.
• Introduce the anthropogenic viewpoint, citing research on the rise of CO2 from industrial processes, deforestation, etc.
• Present the natural cycle perspective, highlighting periods in Earth’s history where temperature fluctuations were observed.
• Discuss overlaps, such as how human activities might exacerbate natural cycles.
• Conclude by emphasising the consensus in the scientific community about human contributions to recent climate change, but acknowledge the existence of natural cycles as part of Earth’s climate history.

Example 2: Merging Historical Texts on a Particular Event

Scenario: You are examining the Battle of Waterloo from British, French, and Prussian primary sources.

• Provide background on the Battle of Waterloo, setting the stage.
• Introduce the British perspective, detailing their strategies, key figures, and their account of the battle’s progression.
• Shift to the French viewpoint, noting their strategic decisions, challenges, and Napoleon’s role.
• Explore the Prussian account, emphasising their contributions and coordination with the British.
• Highlight areas of agreement among the sources (e.g., timeline of events) and areas of discrepancy or unique insights (e.g., differing reasons for the outcome).
• Conclude with a comprehensive view of the battle, incorporating insights from all perspectives and its significance in European history.

Example 3: Synthesising Qualitative And Quantitative Research On A Social Issue

Scenario: You are researching the effects of remote learning on student performance and well-being during the pandemic.

• Start with an introduction to the sudden shift to remote learning due to COVID-19.
• Present quantitative data: statistics showcasing the drop or rise in student grades, attendance rates, and standardised test scores.
• Introduce qualitative insights, like interviews or case studies, highlighting student sentiments, challenges faced at home, or feelings of isolation.
• Discuss the interplay between numbers and narratives. For instance, a drop in grades (quantitative) could be related to a lack of motivation or home distractions (qualitative).
• Compare outcomes across different demographics, using both types of data to show how remote learning might affect diverse student populations differently.
• Conclude with a holistic understanding of the impacts of remote learning, noting areas that need further research or intervention.

Common Mistakes to Avoid when Synthesising Sources

• Relying heavily on a single source limits the depth and breadth of your understanding. It may also inadvertently introduce bias if that source isn’t comprehensive or neutral.
• How to Avoid: Ensure you consult various sources for a well-rounded view. This includes both primary and secondary sources, academic articles, and more accessible pieces like news articles or blogs, if relevant.
• Every source comes with its own perspective. Identifying these biases can lead to a skewed understanding of your topic.
• How to Avoid: Critical reading is key. Always consider who the author is, their potential motivations, the context in which they’re writing, and the methodologies they use.
• Simply presenting what each source says without drawing connections or highlighting contrasts misses the essence of synthesis.
• How to Avoid: As you research, actively look for common themes, conflicting viewpoints, and unique insights. Your goal is to weave a narrative that reflects a comprehensive understanding of all these elements.

Tools and Resources for Synthesising Sources

These are the different tools that can be used for synthesising sources. 

Citation Tools

Managing references can be cumbersome, especially when dealing with numerous sources. Citation tools can help organise, format, and insert citations with ease.

• Zotero: A free, open-source tool that helps you collect, organise, cite, and share research.
• Mendeley: A reference manager and academic social network that can help you organise your research, collaborate with others online, and discover the latest developments.

Mind-Mapping Software or Apps

Mind mapping helps visually organise and interlink ideas, making the synthesis process more intuitive.

• MindMeister: An online mind-mapping tool that lets you capture, develop, and share ideas visually.
• XMind: A popular mind mapping and brainstorming software with various templates to help structure your thoughts.

Note-Taking Apps and Strategies

Effective note-taking is fundamental to understanding and organising information from various sources. Digital note-taking apps often offer features like tagging, search functionalities, and integration with other tools.

• Evernote: A cross-platform app designed for note-taking, organising, and archiving.
• Microsoft OneNote: A digital notebook that allows you to gather drawings, handwritten or typed notes, and save web clippings.
• Cornell Note-taking System: A structured method of note-taking that divides the paper into sections, encouraging active engagement with the material.

Frequently Asked Questions

What is a literature review.

A literature review is a comprehensive survey of existing research on a particular topic, synthesising findings to provide an overview of key concepts, debates, and gaps in knowledge. It establishes a foundation for new research, highlighting relevant studies and contextualising them within the broader academic conversation.

How to synthesise a source?

To synthesise a source, thoroughly understand its content, and then integrate its insights with information from other sources. This involves comparing and contrasting viewpoints, identifying patterns, and constructing a cohesive narrative or argument that offers a broader perspective rather than merely echoing the original source’s content.

Why do I need to cite sources?

Citing sources acknowledges original authors, maintains academic integrity, and provides readers with a reference for verification. It prevents plagiarism by giving credit to the ideas and research of others, allowing readers to trace the evolution of thought and confirm the reliability and accuracy of the presented information.

What are topic sentences?

Topic sentences are the main statements that introduce and summarise a paragraph’s main idea or focus. They provide context and direction, helping readers follow the writer’s argument or narrative. Typically placed at the beginning of a paragraph, they act as signposts, guiding the flow of the discussion.

You May Also Like

In any form of written communication, be it academic writing, journalism, or even casual blogging, there comes a time when we need to reference another’s words to support, explain, or emphasise our points.

A secondary source refers to any material that interprets, analyses, or reviews information originally presented elsewhere. Unlike primary sources, which offer direct evidence or first-hand testimony, secondary sources work on those original materials, offering commentary, critiques, and perspectives.

In today’s information age, where vast amounts of knowledge are easily accessible, it is crucial to know how to use and represent that knowledge correctly and how to cite sources properly.

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How to Write a Literature Review

• 6. Synthesize
• Literature Reviews: A Recap
• Reading Journal Articles
• Does it Describe a Literature Review?
• 1. Identify the Question
• 2. Review Discipline Styles
• Searching Article Databases
• Finding Full-Text of an Article
• Citation Chaining
• When to Stop Searching
• 4. Manage Your References
• 5. Critically Analyze and Evaluate

Synthesis Visualization

Synthesis matrix example.

• 7. Write a Literature Review

• Synthesis Worksheet

About Synthesis

Approaches to synthesis.

You can sort the literature in various ways, for example:

How to Begin?

Read your sources carefully and find the main idea(s) of each source

Look for similarities in your sources – which sources are talking about the same main ideas? (for example, sources that discuss the historical background on your topic)

Use the worksheet (above) or synthesis matrix (below) to get organized

This work can be messy. Don't worry if you have to go through a few iterations of the worksheet or matrix as you work on your lit review!

Four Examples of Student Writing

In the four examples below, only ONE shows a good example of synthesis: the fourth column, or  Student D . For a web accessible version, click the link below the image.

Long description of "Four Examples of Student Writing" for web accessibility

• Download a copy of the "Four Examples of Student Writing" chart

Click on the example to view the pdf.

From Jennifer Lim

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• Last Updated: Jan 10, 2024 4:46 PM
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Synthesizing Research

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When combining another author’s ideas with your own, we have talked about how using the can help make sure your points are being adequately argued (if you have not read our handout on the  evidence cycle,  check it out!). Synthesis takes assertions (statements that describe your claim), evidence (facts and proof from outside sources), and commentary (your connections to why the evidence supports your claim), and blends these processes together to make a cohesive paragraph.

In other words, synthesis encompasses several aspects:

• It is the process of integrating support from more than one source for one idea/argument while also identifying how sources are related to each other and to your main idea.
• It is an acknowledgment of how the source material from several sources address the same question/research topic.
• It is the identification of how important factors (assumptions, interpretations of results, theories, hypothesis, speculations, etc.) relate between separate sources.

TIP: It’s a fruit smoothie!

Think of synthesis as a fruit smoothie that you are creating in your paper. You will have unique parts and flavors in your writing that you will need to blend together to make one tasty, unified drink!

Why Synthesis is Important

• Synthesis integrates information from multiple sources, which shows that you have done the necessary research to engage with a topic more fully.
• Research involves incorporating many sources to understand and/or answer a research question, and discovering these connections between the sources helps you better analyze and understand the conversations surrounding your topic.
• Successful synthesis creates links between your ideas helping your paper “flow” and connect better.
• Synthesis prevents your papers from looking like a list of copied and pasted sources from various authors.
• Synthesis is a higher order process in writing—this is the area where you as a writer get to shine and show your audience your reasoning.

Types of Synthesis

Demonstrates how two or more sources agree with one another.

The collaborative nature of writing tutorials has been discussed by scholars like Andrea Lunsford (1991) and Stephen North (1984). In these essays, they explore the usefulness and the complexities of collaboration between tutors and students in writing center contexts.

Demonstrates how two or more sources support a main point in different ways.

While some scholars like Berlin (1987) have primarily placed their focus on the histories of large, famous universities, other scholars like Yahner and Murdick (1991) have found value in connecting their local histories to contrast or highlight trends found in bigger-name universities.

Accumulation

Demonstrates how one source builds on the idea of another.

Although North’s (1984) essay is fundamental to many writing centers today, Lunsford (1991) takes his ideas a step further by identifying different writing center models and also expanding North’s ideas on how writing centers can help students become better writers.

Demonstrates how one source discusses the effects of another source’s ideas.

While Healy (2001) notes the concerns of having primarily email appointments in writing centers, he also notes that constraints like funding, resources, and time affect how online resources are formed. For writing centers, email is the most economical and practical option for those wanting to offer online services but cannot dedicate the time or money to other online tutoring methods. As a result, in Neaderheiser and Wolfe’s (2009) reveals that of all the online options available in higher education, over 91% of institutions utilize online tutoring through email, meaning these constraints significantly affect the types of services writing centers offer.

Discussing Specific Source Ideas/Arguments

To debate with clarity and precision, you may need to incorporate a quote into your statement. Using can help you to thoroughly introduce your quotes so that they fit in to your paragraph and your argument. Remember that you need to use the to bridge between your ideas and outside source material.

Berlin, J. (1987).  Rhetoric and reality: Writing instruction in American colleges, 1900-1985 . Carbondale: Southern Illinois University Press.

Boquet, E.H. (2001). “Our little secret”: A history of writing centers, pre- to open admissions. In R.W. Barnett & J.S. Blumner (Eds.),  The Allyn and Bacon guide to writing center theory and practice  (pp. 42-60). Boston: Allyn and Bacon.

Carino, P. (1995). Early writing centers: Toward a history.  The Writing Center Journal ,  15 (2), 103-15.

Healy, D. (2001). From place to space: Perceptual and administrative issues in the online writing center. In R.W. Barnett & J.S. Blumner (Eds.), T he Allyn and Bacon guide to writing center theory and practice  (pp. 541-554). Boston: Allyn and Bacon.

Lunsford, A. (1991). Collaboration, control, and the idea of the writing center.  The Writing Center Journal ,  12 (1), 310-75.

Neaderheiser, S. & Wolfe, J. (2009). Between technological endorsement and resistance: The state of online writing centers.  The Writing Center Journal .  29 (1), 49-75.

North, S. (1984). The idea of a writing center.  College English ,  45 (5), 433-446.

Yahner, W. & Murdick, W. (1991). The evolution of a writing center: 1972-1990.  Writing Center Journal ,  11 (2), 13-28.

Literature Review Basics

• What is a Literature Review?
• Synthesizing Research
• Using Research & Synthesis Tables
• Additional Resources

Synthesis: What is it?

First, let's be perfectly clear about what synthesizing your research isn't :

• - It isn't  just summarizing the material you read
• - It isn't  generating a collection of annotations or comments (like an annotated bibliography)
• - It isn't  compiling a report on every single thing ever written in relation to your topic

When you  synthesize  your research, your job is to help your reader understand the current state of the conversation on your topic, relative to your research question.  That may include doing the following:

• - Selecting and using representative work on the topic
• - Identifying and discussing trends in published data or results
• - Identifying and explaining the impact of common features (study populations, interventions, etc.) that appear frequently in the literature
• - Explaining controversies, disputes, or central issues in the literature that are relevant to your research question
• - Identifying gaps in the literature, where more research is needed
• - Establishing the discussion to which your own research contributes and demonstrating the value of your contribution

Essentially, you're telling your reader where they are (and where you are) in the scholarly conversation about your project.

Synthesis: How do I do it?

Synthesis, step by step.

This is what you need to do  before  you write your review.

• Identify and clearly describe your research question (you may find the Formulating PICOT Questions table at  the Additional Resources tab helpful).
• Collect sources relevant to your research question.
• Organize and describe the sources you've found -- your job is to identify what  types  of sources you've collected (reviews, clinical trials, etc.), identify their  purpose  (what are they measuring, testing, or trying to discover?), determine the  level of evidence  they represent (see the Levels of Evidence table at the Additional Resources tab ), and briefly explain their  major findings . Use a Research Table to document this step.
• Study the information you've put in your Research Table and examine your collected sources, looking for  similarities  and  differences . Pay particular attention to  populations ,   methods  (especially relative to levels of evidence), and  findings .
• Analyze what you learn in (4) using a tool like a Synthesis Table. Your goal is to identify relevant themes, trends, gaps, and issues in the research.  Your literature review will collect the results of this analysis and explain them in relation to your research question.

Analysis tips

• - Sometimes, what you  don't  find in the literature is as important as what you do find -- look for questions that the existing research hasn't answered yet.
• - If any of the sources you've collected refer to or respond to each other, keep an eye on how they're related -- it may provide a clue as to whether or not study results have been successfully replicated.
• - Sorting your collected sources by level of evidence can provide valuable insight into how a particular topic has been covered, and it may help you to identify gaps worth addressing in your own work.
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Synthesis: Home

Engaging in synthesis.

Synthesis requires you to make sense of all the relevant ideas in your sources and blend them together with your own thoughts and ideas. Watch this video to learn how to engage in synthesis in order to take research from multiple sources along with your own arguments and turn it into a research paper.

Synthesizing Your Research

Understanding your research.

1. Read through your sources carefully.

2. Identify common themes or sub-topics that keep appearing in the articles you’re reading.

The Research Matrix

• Blank Research Matrix Fill out this blank matrix.
• Blank Synthesis Matrix template (Google Docs)

The research matrix is a helpful tool you can use to synthesize your research along with your own voice. The blank research matrix above can help you organize your paper by main idea, identify connections between your sources, and add your own analysis.

Filling Out Your Matrix

1. Write your topic or research question above the matrix.

2. Write your main ideas for your paper on the left side of the matrix. Helpful Tip: Choose your main ideas AFTER you have read your sources!

3. Write the title, author, or citation of each source in the top row of the matrix.

4. Fill in the matrix boxes with a paraphrase or direct quote that represents how the source discussed that main idea. You do not need every source to address every main idea!

5. Don't forget to nclude your own analysis of the main idea and the sources in the last column on the matrix.

Identify Gaps in Your Research

1. There’s a high likelihood that you will have empty spaces on your research matrix and that’s okay! Small gaps show that there is room for your own voice to join the conversation.

2. Large gaps in your matrix are often a sign that you need to do more research on that main idea. As a rule of thumb you should have at least two sources for each main idea in order to create a meaningful dialogue. 

• Last Updated: Mar 13, 2023 1:12 PM
• URL: https://libguides.usu.edu/synthesizing_info

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Table of Contents

Collaboration, information literacy, writing process, synthesizing your research findings.

• CC BY-NC-ND 4.0 by Christine Photinos - National University, San Diego

Synthesis is something you already do in your everyday life.  For example, if you are shopping for a new car, the research question you are trying to answer is, “Which car should I buy”?  You explore available models, prices, options, and consumer reviews, and you make comparisons.  For example:  Car X costs more than car Y but gets better mileage.  Or:  Reviewers A, B, and C all prefer Car X, but their praise is based primarily on design features that aren’t important to you.  It is this analysis across sources that moves you towards an answer to your question.

Early in an academic research project you are likely to find yourself making initial comparisons—for example, you may notice that Source A arrives at a conclusion very different from that of Source B—but the task of synthesis will become central to your work when you begin drafting your research paper or presentation. 

Remember, when you synthesize, you are not just compiling information.  You are organizing that information around a specific argument or question, and this work—your own intellectual work—is central to research writing.

Below are some questions that highlight ways in which the act of synthesizing brings together ideas and generates new knowledge. 

How do the sources speak to your specific argument or research question?

Your argument or research question is the main unifying element in your project.  Keep this in the forefront of your mind when you write about your sources.  Explain how, specifically, each source supports your central claim/s or suggests possible answers to your question.  For example:  Does the source provide essential background information or a definitional foundation for your argument or inquiry? Does it present numerical data that supports one of your points or helps you answer a question you have posed?  Does it present a theory that might be applied to some aspect of your project?  Does it present a recognized expert’s insights on your topic? 

How do the sources speak to each other? 

Sometimes you will find explicit dialogue between sources (for example, Source A refutes Source B by name), and sometimes you will need to bring your sources into dialogue (for example, Source A does not mention Source B, but you observe that the two are advancing similar or dissimilar arguments).  Attending to interrelationships among sources is at the heart of the task of synthesis.

Begin by asking:  What are the points of agreement?  Where are there disagreements?

But be aware that you are unlikely to find your sources in pure positions of “for” vs. “against.”  You are more likely to find agreement in some areas and disagreement in other areas.  You may also find agreement but for different reasons—such as different underlying values and priorities, or different methods of inquiry.

(See also Identifying a Conversation )

Where are there, or aren’t there, information gaps?

Where is the available information unreliable (for example, it might be difficult to trace back to primary sources), or limited, (for example, based on just a few case studies, or on just one geographical area), or difficult for non-specialists to access (for example, written in specialist language, or tucked away in a physical archive)? 

Does your inquiry contain sub-questions that may not at present be answerable, or that may not be answerable without additional primary research—for example, laboratory studies, direct observation, interviews with witnesses or participants, etc.?

Or, alternatively, is there a great deal of reliable, accessible information that addresses your question or speaks to your argument or inquiry? 

In considering these questions, you are engaged in synthesis: you are conducting an overview assessment of the field of available information and in this way generating composite knowledge.

Remember, synthesis is about pulling together information from a range of sources in order to answer a question or construct an argument. It is something you will be called upon to do in a wide variety of academic, professional, and personal contexts. Being able to dive into an ocean of information and surface with meaningful conclusions is an essential life skill.

Brevity – Say More with Less

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Coherence – How to Achieve Coherence in Writing

Flow – How to Create Flow in Writing

Inclusivity – Inclusive Language

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Summarizing and synthesizing

Part 3: Chapter 10

Questions to consider

A. What distinguishes a synthesis from a summary?

B. How much “author voice” is present relative to source material?

C. What is the nature of the material contributed to a synthesis by the author?

The purpose of synthesizing

Combining separate elements into a whole is the basic dictionary definition of synthesis. It is a way to make connections between numerous and varied source materials. A literature review presents a synthesis of material, grouped by topic, to create a broad and comprehensive view of the literature relevant to a research question. Here, the research questions are often modified to the realities of the information, or information may be selected or rejected based on relevance. This organizational approach helps in understanding the information and structuring the review.

Because research is an iterative process, it is not unusual to go back and search information sources for more material while remaining within the parameters of the topic and research questions. It can be difficult to cope with “everything” on a topic; the need to carefully select based on relevancy is ongoing.

The synthesis must demonstrate a critical analysis of the papers assembled as well as an integration of the analytical results. All included sources must be directly relevant and the synthesis writer should make a significant contribution. As part of an introduction or literature review, the syntheses not only illustrate the evolution of research on an issue, but the writer’s own commentary on what this information means .

Many writers begin the synthesis process by creating a grid, table, or an outline organizing summaries of the source material to discover or extend common themes with the collection. The summary grid or outline provides a researcher an overview to compare, contrast and otherwise investigate the relationships and potential deficiencies. [1]

Language in Action

• How many different sources are used in the synthesis (excerpted from “Does international work experience pay off? The relationship between international work experience, employability and career success: A 30-country, multi-industry study” ) that follows? (IWE: international work experience)
• How do the sources contribute to the message of the paragraph?
• What are the elements of a strong synthesis?
• What information is contributed by the authors themselves?

1 Taking stock of the literature, several characteristics stand out that limit our understanding of the IWE−career success relationship. 2 First, many studies focus on individuals soon after their return from an IWE or while they are still expatriates (Kraimer et al., 2016). 3 These findings may therefore report results pertaining to a short-lived career phase. 4 Given that careers develop over time, and success, especially in the form of promotions and salary increases, may take some time to materialise, it is perhaps not surprising that findings have been mixed. 5 Some authors note that there are short-term, career-related costs of IWE and the career ‘payoff’ occurs after a time lag for which cross-sectional studies may not account (Benson & Pattie, 2008; Biemann & Braakmann, 2013). 6 Second, the majority of studies use samples consisting only of individuals with IWE (Jokinen et al., 2008; Stahl et al., 2009; Suutari et al., 2018). 7 Large samples that include both individuals with and without IWE are needed to provide the variance needed to identify the influence of IWE on career success (e.g., Andresen & Biemann, 2013). 8 Third, studies tend to focus on the baseline question of whether IWE or IWE-specific characteristics (e.g., host country, developmental nature of assignment) are related to a particular career success variable (e.g., Bücker et al., 2016; Jokinen et al., 2008; Stahl et al., 2009). 9 Yet there may be an indirect relationship between IWE and career success (Zhu et al., 2016). 10 More complex models that examine the possible impact of mediating variables are thus needed (Mayrhofer et al., 2012). 11 Lastly, while studies acknowledge that findings from specific countries/nationalities, industries, organisations or occupational roles may not be transferable to all individuals with IWE (Biemann & Braakmann, 2013; Schmid & Wurster, 2017; Suutari et al., 2018), the specific role of national context is rarely considered. 12 However, careers do not develop in a vacuum. 13 Contextual factors play an important role in moderating the career impact of various career experiences such as IWE (Shen et al., 2015). [2]

Organizing the material

Beginning the synthesis process by creating a grid, table, or an outline for summaries of sources offers an overview of the material along with findings and common themes. The summary, grid, or outline will allow quick comparison of the material and reveal gaps in information. [3]

The process of building a “library” from which to draw information is critical in developing the defense, argument or justification of a research study. While field and laboratory research is often engaging and interesting, understanding the backstory and presenting it as an explanation of a proposed method or approach is essential in obtaining funding and/or the necessary committee approval.

Returning to the foundational skill of producing a summary , and combining that with the maintenance of a system to manage source material and details, an annotated bibliography can be both an intellectual structure that reveals connections among sources and a means to initiating – on a manageable level – the arduous writing.

Example – Two entries from an annotated bibliography

Nafisi, A. (2003). Reading Lolita in Tehran: A Memoir in Books. New York: Random House.

A brave teacher in Iran met with seven of her most committed female students to discuss forbidden Western classics over the course of a couple of years, while Islamic morality squads staged raids, universities fell under the control of fundamentalists, and artistic expression was suppressed. This powerful memoir weaves the stories of these women with those of the characters of Jane Austen, F. Scott Fitzgerald, Henry James, and Vladimir Nabokov and extols the liberating power of literature.

Obama, B. (2007). Dreams from My Father. New York: Random House.

This autobiography extends from a childhood in numerous locations with a variety of caregivers (a single parent, grandparents, boarding school) to an exploration of individual heritage and family in Africa, revealing a broken/blended family, abandonment and reconnection, and unresolved endings. Obama describes his existence on the margins of society, the racial tension within his biracial family, and his own identity conflict and turmoil.

Using a chart or grid

Below is a model of a basic table for organizing source material.

Exercise #1

• Read the excerpts from three sources below. Determine the common topic and themes.
• Complete a table like the one above using information from these three sources.

1 Completion of a dissertation is an intense activity. 2 For both groups [completers and non-], the advisor and the student’s family and spouse served as the major source of emotional support and are most heavily invested in the dissertation. 3 Other students and the balance of the dissertation committee were rated as providing little support. 4 Since work on the dissertation is highly individual and there are no College organized groups of students working on the dissertation that meet regularly, the process can be a lonely one. 5 Great independence and a strong sense of direction is required. 6 Although many students rated themselves as having little experience with research, students are dependent on their own resources and on those closest to them. 7 It was noted that graduates rated emotional support from all sources more highly than students rated it. 8 This may be a significant factor associated with dissertation completion.

9 The scales and checklists suggest that there are identifiable differences between the two groups. 10 Since the differences are not great, the implications are that with some modification of procedures, a greater proportion of students can become graduates. 11 Emotional support, financial support, experience with research, familiarity with university and college dissertation requirements, and ready access to university resources and advisors may be factors to build into a modified system to achieve a greater proportion of graduates.

Kluever, R., Green, K. E., Lenz, K., Miller, M. M., & Katz, E. (1995). Graduates and ABDs in colleges of education: Characteristics and implications for the structure of doctoral programs. In  Annual Meeting of the American Educational Research Association. San Francisco, CA. Retrieved from the ERIC database .

1 In this writing group, students evaluated their goal achievement, reflected on the obstacles before them, and set new targets. 2 This process encouraged them to achieve their goals, and they could modify or start a new target instead of giving up. 3 The students also received positive feedback and support from other members of the group. 4 This positive environment helped the students view failure as part of the nature of writing a thesis.

5 On the other hand, daily monitoring encouraged the students to focus more on the process and less on the outcome; therefore, they experienced daily success instead of feeling a failure when the goals were not achievable.

Patria, B., & Laili, L. (2021). Writing group program reduces academic procrastination: a quasi-experimental study.  BMC Psychology ,  9 (1), 1–157. https://doi.org/10.1186/s40359-021-00665-9

1 The promotion of awareness of the tension between core qualities and ideals, and inner obstacles, in particular limiting thoughts, in combination with guidelines for overcoming the tension by being aware of one’s ideals and character strengths is characteristic of the core reflection approach and appears to have a strong potential for diminishing academic procrastination behavior. 2 These results make clear that a positive psychological approach focusing on strengths can be beneficial for diminishing students’ academic procrastination. 3 In particular, supporting and regenerating character strengths can be an effective approach for overcoming academic procrastination.

Visser, L., Schoonenboom, J., & Korthagen, F. A. J. (2017). A Field Experimental Design of a Strengths-Based Training to Overcome Academic Procrastination: Short- and Long-Term Effect. Frontiers in Psychology , 8, 1949–1949. https://doi.org/10.3389/fpsyg.2017.01949

A topical outline is another tool writers may use to organize their material. It begins as a simple list of facts gleaned from various sources and arranged by category. [4]

A topical outline might look like this:

a. fact #1/source #1

b. fact #2/source #1

a. fact #3/source #1

b. fact #4/source #2

Exercise #2

Identify relevant facts presented by the three sources in Exercise #1. Determine the relationships between them. Consider how to categorize and arrange them in order to support or extend a related concept.

Exercise #3

A word about primary sources

Primary source material is information conveyed by the author(s) of the publication. The information they use to support or extend their ideas – their source material – is secondary source material for their readers. Anything considered for inclusion in research writing should be derived from primary sources. When writers find very valuable material cited, they retrieve the original work rather than paraphrase what has already been paraphrased.

Example – Synthesis

The excerpted synthesis below is the work of Joellen E. Coryell, Maria Cinque, Monica Fedeli, Angelina Lapina Salazar, and Concetta Tino. The two primary sources they use in the paragraph were authored by Niehaus and Williams (2016), and Urban, Navarro, and Borron (2017). Because research writers are urged to only use primary sources, further investigation into the paper of Niehaus and Williams would be required in order to use their work as a source. As discussed in Identifying and deploying source material , an effective strategy in finding useful sources is to explore the references of particularly valuable articles or papers.

University Teaching in Global Times: Perspectives of Italian University Faculty on Teaching International Graduate Students

1 Other researchers (Niehaus & Williams, 2016; Urban et al., 2017) offered analyses of faculty’s experiences participating in various training programs for internationalization of their courses.  2 Niehaus and Williams (2016) studied a 4-year global faculty development program aimed at transforming faculty perspectives and internationalizing the curriculum.  3 Findings indicated that participants integrated international and comparative topics to support their learners’ development of global perspectives.  4 They worked to integrate international students’ viewpoints on research, and participants reported professional and personal gains defined by expanded professional networks of faculty members and higher standing that comes with teaching international students.  5 Similarly, Urban et al. (2017) reported findings from a training program that assisted teaching staff to internationalize their courses.  6 The program included a 12-day field trip to a different country.  7 Semi-structured interviews with faculty members, 6 years after participating in the program, affirmed updated course content, new and broader perspectives, and a supportive environment for implementing the internationalized courses and teaching activities.

Primary source:

Coryell, J. E., Cinque, M., Fedeli, M., Lapina Salazar, A., & Tino, C. (2022). University Teaching in Global Times: Perspectives of Italian University Faculty on Teaching International Graduate Students. Journal of Studies in International Education , 26(3), 369–389. https://doi.org/10.1177/1028315321990749

Secondary sources:

Niehaus, E., & Williams, L. (2016). Faculty Transformation in Curriculum Transformation: The Role of Faculty Development in Campus Internationalization. Innovative Higher Education, 41(1), 59–74. https://doi.org/10.1007/s10755-015-9334-7

Urban, E., Navarro, M., & Borron, A. (2017). Long-term Impacts of a Faculty Development Program for the Internationalization of Curriculum in Higher Education. Journal of Agricultural Education, 58(3), 219–238. https://doi.org/10.5032/jae.2017.03219

Experienced researchers often have a strong hypothesis and search for evidence that supports or extends this. However, students often learn about their topic during the research process and formulate a hypothesis as they learn what is established in the field on their topic. Both approaches are acceptable, as is a hybrid.

Discovery phase

Researchers typically begin by paraphrasing any important facts or arguments, tracking their discoveries in a table, outline or spreadsheet. Some good examples include definitions of concepts, statistics regarding relevance, and empirical evidence about the key variables in the research question. The original source information (citations in the appropriate style and format) is as important as the content under consideration.  As shown in the model syntheses here, multiple sources often support a common finding.

Evaluation and analysis phase

A strong synthesis must demonstrate a critical analysis of the papers as well as an integration of analytical results; this is the voice of the synthesis writer, interpreting the relationships of the cited works as they are assembled. Each paper under consideration should be critically evaluated according to its relevancy to the topic and the quality of its content.

Writers first establish relationships between cited concepts and facts by continuously considering these questions:

A. Where are the similarities within each topic or subtopic?

B. Where are the differences?

C. Are the differences methodological or theoretical in nature?

The answers will produce general conclusions for each topic or subtopic as the entire group of studies relate to it.

As the material is organized logically using a grid, table or outline, the most logical order must be determined. That order might be from general to specific, sequential or chronological, or from cause to result. [5]

Review and Reinforce

Summarizing and synthesizing are key building blocks in research writing. Read with an awareness of

A. what information has been added for support;

B. what the source of that information is; and

C. how the information was incorporated (quotations or summaries) and documented (integral or parenthetical citations) into the material.

Research writing is a process itself that synthesizes new information, stylistic tendencies, and established conventions with the background knowledge of the researcher.

Media Attributions

• chameleon © Frontierofficial is licensed under a CC BY (Attribution) license
• 5182866555_18ae623262_c © rarebeasts is licensed under a CC BY (Attribution) license
• Adapted from Frederiksen, L., & Phelps, S. F. (2017). Literature Reviews for Education and Nursing Graduate Students . Open Textbook Library. ↵
• Andresen, M., Lazarova, M., Apospori, E., Cotton, R., Bosak, J., Dickmann, M., Kaše, R., & Smale, A. (2022). Does international work experience pay off? The relationship between international work experience, employability and career success: A 30-country, multi-industry study. Human Resource Management Journal , 32(3), 698–721. https://doi.org/10.1111/1748-8583.12423 ↵
• Adapted from DeCarlo, M. (2018). Scientific Inquiry in Social Work . Open Textbook Library. ↵
• Adapted from DeCarlo, M. (2018). Scientific Inquiry in Social Work. Open Textbook Library. ↵
• Adapted from Frederiksen, L., & Phelps, S. F. (2017). Literature Reviews for Education and Nursing Graduate Students . Open Textbook Library.   ↵

combining separate elements into a whole, generally new, result

a condensed version of a longer text

a list of sources on a particular topic, formatted in the field specific format, which includes a brief summary of each reference

a reference presenting their own data and information

reference material used and cited by a primary source

Sourcing, summarizing, and synthesizing:  Skills for effective research writing  Copyright © 2023 by Wendy L. McBride is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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Chapter 7: Synthesizing Sources

Learning objectives.

At the conclusion of this chapter, you will be able to:

• synthesize key sources connecting them with the research question and topic area.

7.1 Overview of synthesizing

7.1.1 putting the pieces together.

Combining separate elements into a whole is the dictionary definition of synthesis.  It is a way to make connections among and between numerous and varied source materials.  A literature review is not an annotated bibliography, organized by title, author, or date of publication.  Rather, it is grouped by topic to create a whole view of the literature relevant to your research question.

Your synthesis must demonstrate a critical analysis of the papers you collected as well as your ability to integrate the results of your analysis into your own literature review.  Each paper collected should be critically evaluated and weighed for “adequacy, appropriateness, and thoroughness” ( Garrard, 2017 ) before inclusion in your own review.  Papers that do not meet this criteria likely should not be included in your literature review.

Begin the synthesis process by creating a grid, table, or an outline where you will summarize, using common themes you have identified and the sources you have found. The summary grid or outline will help you compare and contrast the themes so you can see the relationships among them as well as areas where you may need to do more searching. Whichever method you choose, this type of organization will help you to both understand the information you find and structure the writing of your review.  Remember, although “the means of summarizing can vary, the key at this point is to make sure you understand what you’ve found and how it relates to your topic and research question” ( Bennard et al., 2014 ).

As you read through the material you gather, look for common themes as they may provide the structure for your literature review.  And, remember, research is an iterative process: it is not unusual to go back and search information sources for more material.

At one extreme, if you are claiming, ‘There are no prior publications on this topic,’ it is more likely that you have not found them yet and may need to broaden your search.  At another extreme, writing a complete literature review can be difficult with a well-trod topic.  Do not cite it all; instead cite what is most relevant.  If that still leaves too much to include, be sure to reference influential sources…as well as high-quality work that clearly connects to the points you make. ( Klingner, Scanlon, & Pressley, 2005 ).

7.2 Creating a summary table

Literature reviews can be organized sequentially or by topic, theme, method, results, theory, or argument.  It’s important to develop categories that are meaningful and relevant to your research question.  Take detailed notes on each article and use a consistent format for capturing all the information each article provides.  These notes and the summary table can be done manually, using note cards.  However, given the amount of information you will be recording, an electronic file created in a word processing or spreadsheet is more manageable. Examples of fields you may want to capture in your notes include:

• Authors’ names
• Article title
• Publication year
• Main purpose of the article
• Methodology or research design
• Participants
• Measurement
• Conclusions

  Other fields that will be useful when you begin to synthesize the sum total of your research:

• Specific details of the article or research that are especially relevant to your study
• Key terms and definitions
• Strengths or weaknesses in research design
• Relationships to other studies
• Possible gaps in the research or literature (for example, many research articles conclude with the statement “more research is needed in this area”)
• Finally, note how closely each article relates to your topic.  You may want to rank these as high, medium, or low relevance.  For papers that you decide not to include, you may want to note your reasoning for exclusion, such as ‘small sample size’, ‘local case study,’ or ‘lacks evidence to support assertion.’

This short video demonstrates how a nursing researcher might create a summary table.

7.2.1 Creating a Summary Table

  Summary tables can be organized by author or by theme, for example:

For a summary table template, see http://blogs.monm.edu/writingatmc/files/2013/04/Synthesis-Matrix-Template.pdf

7.3 Creating a summary outline

An alternate way to organize your articles for synthesis it to create an outline. After you have collected the articles you intend to use (and have put aside the ones you won’t be using), it’s time to identify the conclusions that can be drawn from the articles as a group.

  Based on your review of the collected articles, group them by categories.  You may wish to further organize them by topic and then chronologically or alphabetically by author.  For each topic or subtopic you identified during your critical analysis of the paper, determine what those papers have in common.  Likewise, determine which ones in the group differ.  If there are contradictory findings, you may be able to identify methodological or theoretical differences that could account for the contradiction (for example, differences in population demographics).  Determine what general conclusions you can report about the topic or subtopic as the entire group of studies relate to it.  For example, you may have several studies that agree on outcome, such as ‘hands on learning is best for science in elementary school’ or that ‘continuing education is the best method for updating nursing certification.’ In that case, you may want to organize by methodology used in the studies rather than by outcome.

Organize your outline in a logical order and prepare to write the first draft of your literature review.  That order might be from broad to more specific, or it may be sequential or chronological, going from foundational literature to more current.  Remember, “an effective literature review need not denote the entire historical record, but rather establish the raison d’etre for the current study and in doing so cite that literature distinctly pertinent for theoretical, methodological, or empirical reasons.” ( Milardo, 2015, p. 22 ).

As you organize the summarized documents into a logical structure, you are also appraising and synthesizing complex information from multiple sources.  Your literature review is the result of your research that synthesizes new and old information and creates new knowledge.

7.4 Additional resources:

Literature Reviews: Using a Matrix to Organize Research / Saint Mary’s University of Minnesota

Literature Review: Synthesizing Multiple Sources / Indiana University

Writing a Literature Review and Using a Synthesis Matrix / Florida International University

 Sample Literature Reviews Grid / Complied by Lindsay Roberts

Select three or four articles on a single topic of interest to you. Then enter them into an outline or table in the categories you feel are important to a research question. Try both the grid and the outline if you can to see which suits you better. The attached grid contains the fields suggested in the video .

Literature Review Table  

Test yourself.

• Select two articles from your own summary table or outline and write a paragraph explaining how and why the sources relate to each other and your review of the literature.
• In your literature review, under what topic or subtopic will you place the paragraph you just wrote?

Image attribution

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• Published: 09 April 2024

Navigating phase diagram complexity to guide robotic inorganic materials synthesis

• Jiadong Chen   ORCID: orcid.org/0009-0004-7603-8838 1   na1 ,
• Samuel R. Cross 2   na1 ,
• Lincoln J. Miara   ORCID: orcid.org/0000-0002-2561-8216 2 ,
• Jeong-Ju Cho 2 ,
• Yan Wang   ORCID: orcid.org/0000-0002-8648-2172 2 &
• Wenhao Sun   ORCID: orcid.org/0000-0002-8416-455X 1  

Nature Synthesis ( 2024 ) Cite this article

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• Computational methods
• Design, synthesis and processing

Efficient synthesis recipes are needed to streamline the manufacturing of complex materials and to accelerate the realization of theoretically predicted materials. Often, the solid-state synthesis of multicomponent oxides is impeded by undesired by-product phases, which can kinetically trap reactions in an incomplete non-equilibrium state. Here we report a thermodynamic strategy to navigate high-dimensional phase diagrams in search of precursors that circumvent low-energy, competing by-products, while maximizing the reaction energy to drive fast phase transformation kinetics. Using a robotic inorganic materials synthesis laboratory, we perform a large-scale experimental validation of our precursor selection principles. For a set of 35 target quaternary oxides, with chemistries representative of intercalation battery cathodes and solid-state electrolytes, our robot performs 224 reactions spanning 27 elements with 28 unique precursors, operated by 1 human experimentalist. Our predicted precursors frequently yield target materials with higher phase purity than traditional precursors. Robotic laboratories offer an exciting platform for data-driven experimental synthesis science, from which we can develop fundamental insights to guide both human and robotic chemists.

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Combinatorial synthesis for AI-driven materials discovery

John M. Gregoire, Lan Zhou & Joel A. Haber

There is currently a poor scientific understanding of how to design effective and efficient synthesis recipes to target inorganic materials 1 , 2 , 3 . As a result, synthesis often becomes a bottleneck in the scalable manufacturing of functional materials 4 , as well as in the laboratory realization of computationally predicted materials 5 , 6 . Density functional theory (DFT)-calculated thermodynamic stability or metastability can often estimate the synthesizability of materials 7 , 8 , 9 , but finding an optimal synthesis recipe—including temperatures, times and precursors—still requires extensive trial-and-error experimentation. The recent emergence of robotic laboratories 10 , 11 , 12 , 13 presents an exciting opportunity for high-throughput experiments and sequential learning algorithms to autonomously optimize materials synthesis recipes 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 . However, there remains a poor fundamental understanding of how changing a synthesis recipe affects the underlying thermodynamics and kinetics of a solid-state reaction. Without this scientific foundation, it is difficult to build physics-informed synthesis-planning algorithms to guide robotic laboratories 13 , 23 , meaning that parameter optimization via high-throughput experiments can end up being unnecessarily resource intensive and wasteful.

Multicomponent oxides represent an important and challenging space for targeted synthesis. These high-component materials are key to various device technologies—including battery cathodes (Li(Co,Mn,Ni)O 2 ), oxygen evolution catalysts (Bi 2 Sr 2 Ca n −1 Cu n O 2 n +4+ x ), high-temperature superconductors (HgBa 2 Ca 2 Cu 3 O 8 ) and solid-oxide fuel cells (La 3 SrCr 2 Mn 2 O 12 ) 24 . Multicomponent oxides are usually synthesized by solid-state reactions, which involves combining and firing the constituent binary oxide precursors in a furnace. However, this often yields impurity by-product phases arising from incomplete solid-state reactions. From a phase diagram perspective, precursors start at the corners of a phase diagram and combine together towards a target phase in the interior of the phase diagram. If the phase diagram is complicated, with many competing phases between the precursors and the target, undesired phases may form, consuming thermodynamic driving force and kinetically trapping the reaction in an incomplete non-equilibrium state.

High-component oxides reside in high-dimensional phase diagrams and can be synthesized from many possible precursor combinations. Here we present a thermodynamic strategy to navigate these multidimensional phase diagrams, where the primary objective is to identify precursor compositions that circumvent kinetically competitive by-products, while maximizing the thermodynamic driving force for fast reaction kinetics. We test our principles of precursor selection using a robotic inorganic materials synthesis laboratory, which automates many tedious aspects of the inorganic materials synthesis workflow such as powder precursor preparation, ball milling, oven firing and X-ray characterization of reaction products. With our robotic platform, a single human experimentalist can conduct powder inorganic materials synthesis in both a high-throughput and reproducible manner. Using a diverse target set of 35 quaternary Li-, Na- and K-based oxides, phosphates and borates, which are relevant chemistries for intercalation battery cathodes 25 , 26 and solid-state electrolytes 27 , we show that precursors identified by our thermodynamic strategy frequently outperform traditional precursors in synthesizing high-purity multicomponent oxides. Our work demonstrates the utility of robotic laboratories not only for automated materials synthesis and manufacturing, but also as a platform for large-scale hypothesis validation over a broad and diverse chemical space.

Principles of precursor selection

Recently, we showed that solid-state reactions between three or more precursors initiate at the interfaces between only two precursors at a time 28 . The first pair of precursors to react will usually form an intermediate by-product, which can consume much of the total reaction energy and leave insufficient driving force to complete a reaction 29 . Figure 1 illustrates this multistep reaction progression for an example target compound, LiBaBO 3 , whose simple oxide precursors are B 2 O 3 , BaO and Li 2 CO 3 . Because Li 2 CO 3 decomposes to Li 2 O upon heating, we can examine the competing chemical reactions 30 geometrically on a pseudo-ternary Li 2 O–B 2 O 3 –BaO convex hull. Although the overall reaction energy for Li 2 O + BaO + B 2 O 3  → LiBaBO 3 is large at Δ E  = −336 meV per atom, there are many low-energy ternary phases along the binary slices Li 2 O–B 2 O 3 (Fig. 1b , blue) and BaO–B 2 O 3 (Fig. 1b , green). In the initial pairwise reactions between Li 2 O + BaO + B 2 O 3 , we anticipate that stable ternary Li–B–O and Ba–B–O oxides, such as Li 3 BO 3 , Ba 3 (BO 3 ) 2 or others, will form rapidly due to large thermodynamic driving forces of Δ E  ≈ −300 meV per atom. Should these low-energy intermediates form, the ensuing reaction energies to the target product become miniscule, for example, Li 3 BO 3  + Ba 3 (BO 3 ) 2  → LiBaBO 3 has only Δ E  = −22 meV per atom (Fig. 1e , orange). (Note: one advantage of analysing reactions on convex hulls is that stoichiometric reactions are automatically balanced by the barycentric coordinates of the product relative to its precursors. For brevity, we do not balance reactions explicitly in this manuscript, but we do emphasize that all reaction energies are normalized per atom of product phase.)

a – e , The traditional reaction. f – h , The predicted reaction. Schematic of the pairwise reactions process for traditional recipes ( a ) and our predicted recipes ( f ), showing the phase evolution from precursors to the target. In pseudo-ternary Li 2 O–B 2 O 3 –BaO convex hulls, reaction convex hulls between precursor pairs are illustrated by coloured slices, for B 2 O 3 |Li 2 O (blue) and B 2 O 3 |BaO (green) ( b ), Li 3 BO 3 |Ba 3 (BO 3 ) 2 (orange) ( d ), LiBO 2 |BaO (purple) ( g ). The corresponding two-dimensional slices of the binary reaction convex hulls are B 2 O 3 |Li 2 O (blue) and B 2 O 3 |BaO (green) ( c ), Li 3 BO 3 |Ba 3 (BO 3 ) 2 (orange) ( e ), LiBO 2 |BaO (purple) ( h ), where grey arrows show the reaction energy of the corresponding reaction. i , Free energy change in a reaction progress, where a relatively high-energy intermediate state saves more energy for the final step in forming the target. j , X-ray diffraction of the solid-state synthesis of LiBaBO 3 , where red and blue curves are raw X-ray diffraction data for traditional and predicted precursors, respectively, and the black curve is the fit produced by the Rietveld refinement.

Source data

Instead of allowing the reactions to proceed between the three precursors all at once, we suggest initially synthesizing LiBO 2 , which can serve as a high-energy starting precursor for the reaction. Figure 1g (purple) shows that LiBaBO 3 can be formed directly in the pairwise reaction LiBO 2  + BaO → LiBaBO 3 with a substantial reaction energy of Δ E  = −192 meV per atom. Moreover, along this reaction isopleth, there is a low likelihood of forming impurity phases, as the competing kink of Li 6 B 4 O 9  + Ba 2 Li(BO 2 ) 5 has relatively small formation energy (Δ E  = −55 meV per atom) compared to LiBaBO 3 . Finally, the inverse hull energy of LiBaBO 3 , which we define as the energy below the neighbouring stable phases on the convex hull 31 , is substantial at Δ E inv  = −153 meV per atom, suggesting that the selectivity of the target LiBaBO 3 phase should be much greater than any potential impurity by-products along the LiBO 2 –BaO slice.

Figure 1i compares the energy progression between these two precursor pathways. Although both pathways share the same total reaction energy, synthesizing LiBaBO 3 from three precursors is likely to first produce low-energy ternary oxide intermediates (Fig. 1a ), leaving little reaction energy to drive the reaction kinetics to the target phase 28 . By first synthesizing a high-energy intermediate (LiBO 2 ), we retain a large fraction of overall reaction energy for the last step of the reaction, promoting the rapid and efficient synthesis of the target phase. We confirm this hypothesis experimentally (Fig. 1j ), where we find that solid-state synthesis of LiBaBO 3 from the traditional precursors Li 2 CO 3 , B 2 O 3 and BaO does not result in strong X-ray diffraction signals of the target phase, whereas LiBO 2  + BaO produces LiBaBO 3 with high phase purity ( Methods ).

From this instructive LiBaBO 3 example, we propose five principles to select effective precursors from a multicomponent convex hull. (1) Reactions should initiate between only two precursors if possible, minimizing the chances of simultaneous pairwise reactions between three or more precursors. (2) Precursors should be relatively high energy (unstable), maximizing the thermodynamic driving force and thereby the reaction kinetics to the target phase. (3) The target material should be the deepest point in the reaction convex hull, such that the thermodynamic driving force for nucleating the target phase is greater than all its competing phases. (4) The composition slice formed between the two precursors should intersect as few other competing phases as possible, minimizing the opportunity to form undesired reaction by-products. (5) If by-product phases are unavoidable, the target phase should have a relatively large inverse hull energy—in other words, the target phase should be substantially lower in energy than its neighbouring stable phases in composition space.

When there were multiple precursor pairs that could be used to synthesize the target compound, we ranked the ‘best’ precursor pair by first prioritizing principle 3, where the target compound was at the deepest point of the convex hull. This ensures that the thermodynamic driving force for nucleation of the target compound is greater than the driving forces to all other competing phases. We next prioritized principle 5, where the target compound has the largest inverse hull energy. A reaction having a large inverse hull energy supersedes principle 2, as a large reaction driving force is not a sufficient criterion for synthesis, for example, in Fig. 2b , where the magnitude of the driving force of Li 2 O + Zn 2 P 2 O 7  → LiZnPO 4 is large but selectivity may be weak compared to ZnO + Li 3 PO 4 . A large inverse hull energy also supersedes principle 4, as a large inverse hull energy means that, even if intermediate phases form, there would still be a large driving force for a secondary reaction to form the target compound.

a , c , e , The blue, red and purple slice planes correspond to Zn 2 P 2 O 7  + Li 2 O ( a ), Zn 3 (PO 4 ) 2  + Li 3 PO 4 ( c ) and LiPO 3  + ZnO ( e ) binary reaction convex hulls, respectively. b , d , f , The corresponding two-dimensional slices of reaction convex hulls are shown for Zn 2 P 2 O 7  + Li 2 O ( b ), Zn 3 (PO 4 ) 2  + Li 3 PO 4 ( d ) and LiPO 3  + ZnO ( f ).

In Fig. 2 , we interpret these precursor design principles for an example LiZnPO 4 target in the pseudo-ternary Li 2 O–P 2 O 5 –ZnO phase diagram. If we first synthesize Zn 2 P 2 O 7 to combine with Li 2 O (Fig. 2a,b , blue), the deepest point in the reaction convex hull is not LiZnPO 4 but rather ZnO + Li 3 PO 4 , suggesting a kinetic propensity to form these undesired by-products. If we start from Zn 3 (PO 4 ) 2  + Li 3 PO 4 (Fig. 2c,d , orange), LiZnPO 4 is located at the deepest point along the convex hull; however, Li 3 PO 4 is a low-energy starting precursor, meaning that there is a small driving force (Δ E  = −40 meV per atom) left to form LiZnPO 4 , which probably leads to slow reaction kinetics. We suggest that LiPO 3  + ZnO (Fig. 2e,f , purple) are the ideal precursors for LiZnPO 4 . LiPO 3 has a relatively high energy along the Li 2 O–P 2 O 5 binary hull, resulting in a large driving force to the target phase of Δ E  = −106 meV per atom. Additionally, there are no competing phases along the LiPO 3  + ZnO slice, minimizing the possibility of impurity by-product phases.

In Supplementary Note 1.2 , we further interpret our precursor selection principles from the dual perspective of chemical potential diagrams, and interpret the inverse hull energy with respect to the ‘chemical potential distance’, as proposed by Todd et al. 32 . Here, we chose a convex hull approach, since it graphically constrains stoichiometrically balanced pairwise reactions better than chemical potential diagrams. Additionally, in Supplementary Note 1.3 , we show that our predicted precursors generally differ from those predicted by the algorithms of McDermott et al. 30 and Aykol et al. 33 . Although all our works share the same goal of predicting inorganic synthesis recipes, the five principles that guide our precursor selection algorithm are based on our recent insights into the importance of pairwise reactions 27 , 28 , which was not considered in the PIRO algorithm by Aykol et al. PIRO therefore predicts the optimal precursors for BaLiBO 3 to be \(\frac12\) Ba +  \(\frac12\) Ba(BO 2 ) 2  + Li +  \(\frac12\) O 2  → BaLiBO 3 , which probably proceeds through intermediates in this multiprecursor reaction. Our approach of maximizing driving force also differs slightly from the cost function of McDermott et al., whose ideal predicted reaction is \(\frac13\) Ba 3 (BO 3 ) 2  +  \(\frac13\) Li 3 BO 3  → BaLiBO 3 , which, as discussed earlier, has a small driving force. As deeper fundamental understanding of solid-state reactions is achieved, we anticipate that new principles will need to be developed and included in our algorithms for the overarching ambition of predictive solid-state synthesis.

Validation with a robotic ceramic synthesis laboratory

To test our precursor selection hypotheses, we designed a large-scale experimental validation effort based on quaternary Li-, Na- and K-based oxides, phosphates and borates, which are representative chemistries for intercalation battery materials 24 , 26 . We surveyed the Materials Project 34 for all known quaternary compounds in this space, and then we used our selection principles to predict optimal precursors from the DFT-calculated convex hulls. The algorithms to identify these precursors are detailed in Supplementary Note 1 . We also determine the traditional precursors for these reactions, which we previously text-mined from the solid-state synthesis literature 35 . A full list of 3,104 reactions in this space is provided in Supplementary Data 1 . To efficiently maximize the coverage of our experimental validation, we Pareto-optimized our reaction list to select the fewest number of precursors that maximize the number of candidate reactions, resulting in 28 unique precursors for 35 target materials that span 27 elements.

We then compare the phase purity of target materials synthesized from our predicted precursors versus that from traditional precursors. We perform this large-scale validation effort using a robotic inorganic materials synthesis laboratory named ASTRAL (Automated Synthesis Testing and Research Augmentation Lab), located at the Samsung Advanced Institute of Technology, Cambridge, Massachusetts. As shown in Fig. 3 , ASTRAL uses a robotic arm to automate sample handling throughout a full ceramic synthesis workflow—from powder precursor preparation to ball milling, to oven firing and to X-ray characterization of reaction products. Videos of the robotic laboratory in action can be viewed in Supplementary Videos 1 – 4 . Three trays of 24 samples can pass sequentially through the ASTRAL workflow every 72 hours. The throughput of ASTRAL is bottlenecked by powder dispensing and processing, as each 24-sample tray is prepared serially, whereas the firing and characterization steps can, in principle, be run in parallel.

a , A robot-enabled inorganic materials synthesis workflow—from powder precursor preparation to ball milling, to oven firing and to X-ray diffraction (XRD) characterization of reaction products. b , Photograph of the ASTRAL laboratory. c , Robotic chemists enable large-scale exploration of synthesis hypotheses over a broad chemical space, which normally would have to be undertaken by multiple experimentalist groups. d , Human experimentalists have a trade-off between throughput and reproducibility, whereas robotic chemists can achieve both high reproducibility and throughput simultaneously.

ASTRAL automates inorganic materials synthesis from powder precursors, as opposed to previous robotic laboratories that rely on solution-based precursors 15 , 16 , 17 , 36 , inkjet printing 18 or combinatorial thin-film deposition 14 , 19 . Although it is easier to dose precursor concentrations using these other methods, the resulting products are typically only produced at milligram scale. Powder synthesis, on the other hand, can yield grams of material, which is needed to create ceramic pellets or electrodes for functional property characterization. Moreover, high-temperature powder synthesis is the primary synthesis method of ceramic oxides, so recipes determined from ASTRAL can be upscaled for industrial manufacturing. We overcame major practical challenges in powder precursor processing, which arise primarily from flowability differences between different powders due to varying particle sizes, hardness, hygroscopicity and compaction. In Supplementary Table 1 , we summarize the challenges in working with powder precursors, as well as our solutions to these challenges. The essential task is to identify the best dosing head for each precursor, as detailed in Supplementary Table 2 for the precursors used here.

In total, we conducted 224 synthesis reactions over 35 target materials, calcined at temperatures from 600 to 1,000 °C. Each reaction was conducted for 8 hours, and then impurity by-products were assessed, without regrinding or reannealing our samples. We deliberately chose these relatively short reaction times to evaluate the intrinsic reactivity of the two competing sets of precursors.

For a target space this diverse, traditional validation of our precursor selection principles would probably have required an extensive experimental effort, consisting of multiple human experimentalists working over many years. Once the robotic laboratory is set up, we can comprehensively survey this broad crystal chemistry space in a single experimental campaign (Fig. 3c ). Moreover, a large-scale human effort will inevitably require trade-offs between throughput and reproducibility. Meanwhile, a robotic laboratory produces single-source experimental data with high reproducibility, meaning we can systematically compare synthesis results while minimizing human variability and error (Fig. 3d ). Altogether, the robotic laboratory offers a new platform for data-driven empirical synthesis science, where hypotheses can be investigated rapidly, reproducibly and comprehensively over diverse crystal chemistries.

Results and discussion

For the 35 materials selected, Fig. 4a shows the relative yield of the target phase starting from computationally designed versus traditional precursors. Figure 4b shows the reaction temperatures attempted and Fig. 4c shows the relative performance of the predicted versus traditional precursors. A full list of targets, precursors and reaction results is given in Supplementary Table 3 . For 32 out of 35 compounds (91%), the predicted precursors successfully produced the target phase. In 15 targets, the predicted precursors achieved at least 20% higher phase purity than the traditional precursors (green), and 6 of these 15 target materials could ‘only’ be synthesized by the predicted precursors (dark green). For 16 reactions the precursors have similar target yields (light green), and only in 4 systems do the traditional precursors perform better than the predicted precursors (red). However, we note that even in these four systems, the predicted precursors also produce the target materials with moderate to high purities.

a , Table of the phase purity of 35 targets obtained from predicted precursors using the highest phase purity from various firing temperatures, compared to traditional precursors. Colour of ‘Precursor comparison’ column compares purity from predicted precursors versus traditional precursors, where green means predicted precursors achieve >10% better purity, light green means they have purities within ±10% and red means traditional precursors achieve >10% better purity. Targets with blue colour star are metastable materials. The same colour scheme is used in panels b – d . b , Heat map of phase purity of predicted precursors at different calcination temperatures. c , The target phase purity from predicted precursors versus traditional precursors. Phase purity methods in Supplementary Note 2.2.3 . d , Reaction energies and inverse hull energies for all targets. The marker shape corresponds to the best phase purity of predicted precursors, where diamonds are high purity, circles are moderate and low purity, and crosses with a red outline mean that both predicted precursors and traditional precursors failed. The red box represents the low thermodynamic driving force regions where kinetic process may be rate limiting. The dashed line represents when the inverse hull energy equals the reaction energy. Inset, convex hull illustrating the reaction energy and the inverse hull energy.

We also examined the robotic solid-state synthesis of four metastable compounds with mild energies above the convex hull 7 —LiNbWO 6 (10 meV per atom), LiZnBO 3 (8 meV per atom), KTiNbO 5 (1 meV per atom) and Li 3 Y 2 (BO 3 ) 3 (39 meV per atom), indicated by blue asterisks in Fig. 4 . These metastable compounds formed in our solid-state reactions, although generally with low phase purity. However, we still found that our predicted precursors would yield these target metastable phases with similar or better relative purity than when starting from traditional precursors (Supplementary Note 3.2 ). Recent work by Zeng et al. suggests that by tuning the thermodynamic driving forces from the precursors, it may be possible to selectively form desired stable or metastable phases on the basis of their calculated nucleation barriers 37 . Finally, in three systems, neither sets of precursors resulted in the target material, which for NaBSiO 4 was due to glass formation 38 , for Li 3 V 2 (PO 4 ) 3 a more reducing atmosphere was needed 39 and for NaBaBO 3 the published reaction temperature 40 was very precise at 790 °C, suggesting that perhaps a rounded number, such as 800 °C, may be too high. As discussed further in Supplementary Note 3.4 , these potential failure modes represent important considerations in future robotic laboratory design for solid-state synthesis.

Figure 4c shows that our predicted precursors tend to synthesize target materials with higher purity than traditional simple oxide precursors. Many of our predicted ternary oxide precursors are unusual, such as LiPO 3 , LiBO 2 and LiNbO 3 (see more in Supplementary Table 3 ), as these precursors do not appear from our previously text-mined database of 19,488 solid-state synthesis recipes 41 . Machine-learning algorithms for synthesis prediction trained on literature datasets would therefore be unlikely to predict our suggested precursors here. This highlights the limitations of machine-learning algorithms in predicting new opportunities in synthesis parameter space, outside the constraints of our anthropogenic biases in chemical reaction data 22 , 42 .

Our results show that the success of a reaction was not correlated to the crystal structure or chemistry of the target material; rather, it was primarily determined by the geometry of the underlying convex hull, as well as by the magnitude of the thermodynamic driving force. The success of our precursor selection principles is surprising, considering we evaluate precursor selection using only the DFT-calculated convex hull, which does not account for temperature-dependent effects, such as vibrational entropy or oxide decomposition, neglects kinetic considerations, such as diffusion rates and nucleation barriers 32 , and has known errors in DFT-calculated formation energies 43 .

Here we rationalize with order-of-magnitude energy arguments why, despite many simplifying assumptions, the DFT-calculated thermodynamic convex hull retains predictive power in identifying effective precursors. First, entropic contributions ( T Δ S ; T is temperature and S is entropy) can generally be neglected because the free energy change Δ G of an oxide synthesis reaction is usually dominated by the change in enthalpy Δ H contribution, rather than the T Δ S contribution. Supplementary Fig. 16 compiles a list of 100 experimental ternary oxide reaction energies, and shows that at 1,000 K the magnitude of |Δ G | for reactions is ~200 meV per atom, whereas the | T Δ S | contribution is only ~15 meV per atom. In 60% of the reactions, | T Δ S |/|Δ G | < 10%, except in cases where |Δ G | < 100 meV per atom, in which case T Δ S can be comparable in magnitude to Δ H . We validate these arguments in Supplementary Fig. 17 , showing that temperature-dependent free energies are negligibly different from reaction enthalpies 44 . The dominance of Δ H over T Δ S in oxide synthesis reactions is due to the irreversible exothermic nature of reactions of the form A + B → AB; as opposed to first-order phase transitions, such as melting, or polymorphic transformations, where Δ H  ≈  T Δ S . This assumption relies on both the reactants and products being solid phases—for reactions that evolve gases, the reaction entropy is approximately Δ S  = 1 eV per atom per 1,000 K; meaning that higher temperature largely favours the reaction direction with more moles of gas.

Second, ternary convex hulls are often skewed such that certain hull directions are much deeper than others, such as the Li 2 O–B 2 O 3 and BaO–B 2 O 3 directions illustrated on the Li 2 O–BaO–B 2 O 3 convex hull in Fig. 1 (more examples can be found in Supplementary Note 3 ). On a high-dimensional phase diagram, there are many combinations of precursor pairs that can slice through a target phase. Even an approximate convex hull, with systematic DFT formation energy errors of 25 meV per atom (refs. 8 , 42 ), can largely capture the relative depths of the convex hull in various compositional directions, as well as the complexity of the hull arising from competing phases. Importantly, DFT is well poised to capture the very stable phases, which are low-energy thermodynamic sinks to be avoided when designing the reaction isopleths between pairs of precursors.

Although we do not explicitly calculate kinetics here, the magnitude of the thermodynamic driving force is a good proxy for phase transformation kinetics, as Δ G reaction appears in the denominator of the classical nucleation barrier, as supersaturation in the JMAK theory of crystal growth and as d μ /d x in Fick’s first law of diffusion (where μ is chemical potential and x is distance) 45 . Because we aim to evaluate the relative reaction kinetics of different precursors, rather than absolute kinetics, we can usually compare thermodynamic driving forces between different precursor sets without explicitly calculating diffusion barriers 46 or surface energies for nucleation and growth analyses 47 , 48 .

However, there are limits to this assumption. Figure 4d shows the reaction energy and inverse hull energy for all 35 reactions using predicted precursors, among which three of the unsuccessful syntheses are marked with a cross and four red markers indicate conditions where the traditional precursors outperformed the predicted precursors. In cases where our predicted precursors were less successful (red box in Fig. 4d ), the reaction energy landscapes were shallow with Δ E reaction  > −70 meV per atom, and inverse hull energies of Δ E IH  > −50 meV per atom. Because these driving forces are of the order of k B T at solid-state synthesis temperatures (Boltzmann’s constant × ~1,000 K ≈ 100 meV per atom), unanticipated kinetic processes may become rate limiting and disqualify our thermodynamic driving force arguments. These counter examples provide valuable ‘failed synthesis’ results 49 to quantify bounds where our precursor selection principles offer less certainty of success, and can serve as soft cutoff energies for future algorithms for solid-state precursor prediction—although we note that many reactions within this energy cutoff can still be successful, as shown in our experiments.

Finally, additional opportunities to design large Δ G reaction include leveraging metathesis reactions 29 , 31 , for example, of the form 2NaCrS 2  + MgCl 2  → MgCr 2 S 4  + 2NaCl (ref. 50 ), where reactions can be thermodynamically driven by the formation of a stable salt by-product. Because there are a wide variety of opportunities to select potential by-product phases, metathesis reactions represent a rich design space to enhance the thermodynamics, and thereby the kinetics, of solid-state reactions.

Synthesis science is poorly understood, but new theories can be developed by examining falsifiable predictions through empirical validation. In this work, we hypothesized several principles to identify superior precursors for high-purity synthesis of multicomponent oxides. We argued that in high-dimensional phase diagrams with skewed energy landscapes, there is an opportunity to find precursors that are both high in energy and have compositions that circumvent low-energy, undesired kinetic by-products. Using a robotic synthesis laboratory, we validated this hypothesis over 35 target materials with diverse crystal chemistries, producing in this one study as many experimental results as a typical review paper might survey. This work highlights the potential of data-driven experimental synthesis science, where the high throughput and reproducibility of robotic laboratories enable a more comprehensive interrogation of synthesis science hypotheses. This exciting robotic platform can be directed to investigate further fundamental questions, such as the role of temperatures and reaction times in ceramic oxide synthesis. As we use these robotic laboratories to verify human-designed hypotheses, we will deepen our fundamental understanding of the interplay between thermodynamics and kinetics during materials formation. Simultaneously, this scientific understanding will drive the development of physically informed artificial intelligence synthesis-planning frameworks to enable truly autonomous materials processing and manufacturing.

DFT convex hulls for precursor identification

Material phases and formation energies were obtained from the Materials Project 34 using its REST API 51 (retrieved December 2020). Convex hulls were constructed from the phase diagram module in Pymatgen 52 , and reaction convex hulls were calculated from the interfacial reactions module 53 . Software for producing interactive reaction compound convex hulls can be found on GitHub at https://github.com/dd-debug/synthesis_planning_algorithm . Further details on the thermodynamic calculations and convex hull analysis are provided in Supplementary Note 1 .

Robotic laboratory

ASTRAL transports samples between stations using two robots, a seven-axis Panda robotic arm (Franka Emika) and a linear rail (Vention). By using the rail system to extend the range of the Panda arm, the system can perform precise laboratory manipulations over an area of 1.7 m × 4 m. Surrounding the central rail system are stations that perform specialized tasks for inorganic materials synthesis, such as dispensing solid powder precursor chemicals and liquid dispersants, a mechanical ball mill, furnace to calcine and react precursors and X-ray diffraction to characterize synthesis outcomes. Precursor powders are dispensed sequentially using a Quantos powder dispenser (Mettler Toledo), with sample vials and powder dosing heads exchanged using the robotic arm. Following powder dispensing, 1 ml of ethanol is dispensed into each vial using a Freedom EVO 150 liquid handling robot (Tecan Life Sciences), followed by rotary ball milling for 15 h at 100 r.p.m. to produce a uniform fine mixture of precursor powders. Alumina crucibles (Advalue Technology) are used to hold the mixed precursors. After ball milling, samples are heated to 80 °C for 2 h under vacuum to remove ethanol and then transferred to a furnace for calcination in air atmosphere for 8 h at temperatures from 600 to 1,000 °C. Powders are then characterized via powder X-ray diffraction (Rigaku Miniflex 600). Further details on the robotic infrastructure and synthesis procedures are provided in Supplementary Note 2 .

Automated X-ray diffraction refinement

Rietveld refinement of data was performed in the BGMN kernel 54 . The target structure is used as the sole input phase for the BMGN kernel, and the Rietveld refinement will split the X-ray diffraction signal into the target phase, background and residual. The background X-ray diffraction pattern was determined from empty sample holders. The fraction of the target phase was estimated by dividing the integrated intensity ( I ) of the target phase by the combined intensity of the target phase and residual phase, I target /( I target  +  I residual ). Values greater than 0.5 are considered high purity, between 0.2 and 0.5 are considered moderate purity and less than 0.2 is considered low purity. Further details on the automated X-ray diffraction refinement process are provided in Supplementary Note 2.2.3 .

Data availability

The data supporting the findings of this study are available within the paper and its Supplementary Information files. All thermodynamic data to reproduce our analyses can be freely obtained from the Materials Project database and its API, as discussed in Methods and Supplementary Note 1 . X-ray diffraction patterns for robotic laboratory synthesis results are all provided in the Supplementary Information . All experimental protocols regarding the construction and operation of the robotic laboratory are discussed in the Supplementary Information . Candidate reactions and their energies are available via figshare at https://doi.org/10.6084/m9.figshare.22671571 . Source data are provided with this paper.

Code availability

All code for evaluating precursors, as well as for producing interactive reaction compound convex hulls, can be found on GitHub at the following link: https://github.com/dd-debug/synthesis_planning_algorithm . The link includes a readme, demonstration file, installation guide, Python package requirements and instructions for use.

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Acknowledgements

This work was supported by the US Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under award no. DE-SC0021130. We thank J. Morgan for contributions to the development of Samsung ASTRAL. W.S. thanks S. Y. Chan for important discussions and support.

Author information

These authors contributed equally: Jiadong Chen, Samuel R. Cross.

Authors and Affiliations

Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI, USA

Jiadong Chen & Wenhao Sun

Advanced Materials Lab, Samsung Advanced Institute of Technology–America, Samsung Semiconductor Inc., Cambridge, MA, USA

Samuel R. Cross, Lincoln J. Miara, Jeong-Ju Cho & Yan Wang

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Contributions

J.C. and W.S. developed precursor selection principles, analysed candidate synthesis reactions and predicted optimal synthesis precursors for experimental validation. S.R.C., L.J.M. and Y.W. designed the traditional synthesis precursors. S.R.C., L.J.M., J.-J.C. and Y.W. built the ASTRAL automated laboratory. S.R.C. synthesized and characterized the quaternary materials. J.C., S.R.C. and W.S. conducted phase purity analysis. J.C., S.R.C., Y.W. and W.S. wrote the manuscript and Supplementary Information, with contributions and revisions from all authors. W.S., L.J.M. and Y.W. conceived and supervised all of the main aspects of the project.

Corresponding authors

Correspondence to Yan Wang or Wenhao Sun .

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The authors declare no competing interests.

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Nature Synthesis thanks Milad Abolhasani and Kedar Hippalgaonkar for their contribution to the peer review of this work. Primary Handling Editor: Alexandra Groves, in collaboration with the Nature Synthesis team.

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Supplementary information

Supplementary information.

Supplementary Notes 1–3, including Supplementary Figs. 1–52, Discussion and Tables 1–75.

Supplementary Data 1

Additional 3,104 predicted reactions.

Supplementary Video 1

ASTRAL dispensing power part 1.

Supplementary Video 2

ASTRAL dispensing power part 2.

Supplementary Video 3

ASTRAL furnace workflow.

Supplementary Video 4

ASTRAL XRD workflow.

Supplementary Data 2

Raw data for all X-ray diffraction figures in the Supplementary Information.

Source Data Fig. 1

Raw X-ray diffraction data for Fig. 1j.

Source Data Fig. 4

Statistical source data for Fig. 4c,d.

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Chen, J., Cross, S.R., Miara, L.J. et al. Navigating phase diagram complexity to guide robotic inorganic materials synthesis. Nat. Synth (2024). https://doi.org/10.1038/s44160-024-00502-y

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Received : 25 April 2023

Accepted : 16 February 2024

Published : 09 April 2024

DOI : https://doi.org/10.1038/s44160-024-00502-y

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