type of research evidence

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Evidence-Based Research: Evidence Types

Introduction.

Not all evidence is the same, and appraising the quality of the evidence is part of evidence-based practice research. The hierarchy of evidence is typically represented as a pyramid shape, with the smaller, weaker and more abundant research studies near the base of the pyramid, and systematic reviews and meta-analyses at the top with higher validity but a more limited range of topics.

Several versions of the evidence pyramid have evolved with different interpretations, but they are all comprised of the types of evidence discussed on this page. Walden's Nursing 6052 Essentials of Evidence-Based Practice class currently uses a simplified adaptation of the Johns Hopkins model .

Evidence Levels:

Level I:  Experimental, randomized controlled trial (RCT), systematic review RTCs with or without meta-analysis

Level II:  Quasi-experimental studies, systematic review of a combination of RCTs and quasi-experimental studies, or quasi-experimental studies only, with or without meta-analysis

Level III:  Nonexperimental, systematic review of RCTs, quasi-experimental with/without meta-analysis, qualitative, qualitative systematic review with/without meta-synthesis  (see Daly 2007 for a sample qualitative hierarchy) 

Level IV : Respected authorities’ opinions, nationally recognized expert committee or consensus panel reports based on scientific evidence

Level V:  Literature reviews, quality improvement, program evaluation, financial evaluation, case reports, nationally recognized expert(s) opinion based on experiential evidence

Systematic review

What is a systematic review.

A systematic review is a type of publication that addresses a clinical question by analyzing research that fits certain explicitly-specified criteria. The criteria for inclusion is usually based on research from clinical trials and observational studies. Assessments are done based on stringent guidelines, and the reviews are regularly updated. These are usually considered one of the highest levels of evidence and usually address diagnosis and treatment questions.

Benefits of Systematic Reviews

Systematic reviews refine and reduce large amounts of data and information into one document, effectively summarizing the evidence to support clinical decisions. Since they are typically undertaken by a entire team of experts, they can take months or even years to complete, and must be regularly updated. The teams are usually comprised of content experts, an experienced searcher, a bio-statistician, and a methodologist. The team develops a rigorous protocol to thoroughly locate, identify, extract, and analyze all of the evidence available that addresses their specific clinical question.

As systematic reviews become more frequently published, concern over quality led to the PRISMA Statement to establish a minimum set of items for reporting in systematic reviews and meta-analyses.

Many systematic reviews also contain a meta-analysis.

What is a Meta-Analysis?

Meta-analysis is a particular type of systematic review that focuses on selecting and reviewing quantitative research. Researchers conducting a meta-analysis combine the results of several independent studies and reviews to produce a synthesis where possible. These publications aim to assist in making decisions about a particular therapy.

Benefits of Meta-Analysis

A meta-analysis synthesizes large amounts of data using a statistical examination. This type of analysis provides for some control between studies and generalized application to the population.

To learn how to find systematic reviews in the Walden Library, please see the Levels of Evidence Pyramid page:

• Levels of Evidence Pyramid: Systematic Reviews

Further reading

• Cochrane Handbook for Systematic Reviews of Interventions *updated 2022

Guidelines & summaries

Practice guidelines.

A practice guideline is a systematically-developed statement addressing common patient health care decisions in specific clinical settings and circumstances.  They should be valid, reliable, reproducible, clinically applicable, clear and flexible. Documentation must be included and referenced. Practice guidelines may come from organizations, associations, government entities, and hospitals/health systems.

ECRI Guidelines Trust

Best Evidence Topics

Best evidence topics are sometimes referred to as Best BETs. These topics are developed and supported for situations or setting when the high levels of evidence don't fit or are unavailable. They originated from emergency medicine providers' need to conduct rapid evidence-based clinical decisions.

Critically-Appraised Topics

Critically-appraised topics are a standardized one- to two-page summary of the evidence supporting a clinical question. They include a critique of the literature and statement of relevant results. They can be found online in many repositories.

To learn how to find critically-appraised topics in the Walden Library, please see the Levels of Evidence Pyramid page:

• Levels of Evidence Pyramid: Critically-Appraised Topics

Critically-Appraised Articles

Critically-appraised articles are individual articles by authors that evaluate and synopsize individual research studies. ACP Journal Club is the most well known grouping of titles that include critically appraised articles.

To learn how to find critically-appraised articles in the Walden Library, please see the Levels of Evidence Pyramid page:

• Levels of Evidence Pyramid: Critically-Appraised Articles

Randomized controlled trial

A randomized controlled trial (RCT) is a clinical trial in which participants are randomly assigned to either the treatment group or control group. This random allocation of participants helps to reduce any possible selection bias and makes the RCT a high level of evidence. Having a control group, which receives no treatment or a placebo treatment, to compare the treatment group against allows researchers to observe the potential efficacy of the treatment when other factors remain the same. Randomized controlled trials are quantitative studies and are often the only studies included in systematic reviews.

To learn how to find randomize controlled trials, please see our CINAHL & MEDLINE help pages:

• CINAHL Search Help: Randomized Controlled Trials
• MEDLINE Search Help: Randomized Controlled Trials

Cohort study

A cohort study is an observational longitudinal study that analyzes risk factors and outcomes by following a group (cohort) that share a common characteristic or experience over a period of time.

Cohort studies can be retrospective, looking back over time at data that has already been collected, or can be prospective, following a group forward into the future and collecting data along the way.

While cohort studies are considered a lower level of evidence than randomized controlled trials, they may be the only way to study certain factors ethically. For example, researchers may follow a cohort of people who are tobacco smokers and compare them to a cohort of non-smokers looking for outcomes. That would be an ethical study. It would be highly unethical, however, to design a randomized controlled trial in which one group of participants are forced to smoke in order to compare outcomes.

To learn how to find cohort studies, please see our CINAHL and MEDLINE help pages:

• CINAHL Search Help: Cohort Studies
• MEDLINE Search Help: Cohort Studies

Case-controlled studies

Case-controlled studies are a type of observational study that looks at patients who have the same disease or outcome. The cases are those who have the disease or outcome while the controls do not. This type of study evaluates the relationship between diseases and exposures by retrospectively looking back to investigate what could potentially cause the disease or outcome.

To learn how to find case-controlled studies, please see our CINAHL and MEDLINE help pages:

• CINAHL Search Help: Case Studies
• MEDLINE Search Help: Case Studies

Background information & expert opinion

Background information and expert opinion can be found in textbooks or medical books that provide basic information on a topic. They can be helpful to make sure you understand a topic and are familiar with terms associated with it.

To learn about accessing background information, please see the Levels of Evidence Pyramid page:

• Levels of Evidence Pyramid: Background Information & Expert Opinion
• Previous Page: Levels of Evidence Pyramid
• Next Page: CINAHL Search Help
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• Research Process

Levels of evidence in research

• 5 minute read

Table of Contents

Level of evidence hierarchy

When carrying out a project you might have noticed that while searching for information, there seems to be different levels of credibility given to different types of scientific results. For example, it is not the same to use a systematic review or an expert opinion as a basis for an argument. It’s almost common sense that the first will demonstrate more accurate results than the latter, which ultimately derives from a personal opinion.

In the medical and health care area, for example, it is very important that professionals not only have access to information but also have instruments to determine which evidence is stronger and more trustworthy, building up the confidence to diagnose and treat their patients.

5 levels of evidence

With the increasing need from physicians – as well as scientists of different fields of study-, to know from which kind of research they can expect the best clinical evidence, experts decided to rank this evidence to help them identify the best sources of information to answer their questions. The criteria for ranking evidence is based on the design, methodology, validity and applicability of the different types of studies. The outcome is called “levels of evidence” or “levels of evidence hierarchy”. By organizing a well-defined hierarchy of evidence, academia experts were aiming to help scientists feel confident in using findings from high-ranked evidence in their own work or practice. For Physicians, whose daily activity depends on available clinical evidence to support decision-making, this really helps them to know which evidence to trust the most.

So, by now you know that research can be graded according to the evidential strength determined by different study designs. But how many grades are there? Which evidence should be high-ranked and low-ranked?

There are five levels of evidence in the hierarchy of evidence – being 1 (or in some cases A) for strong and high-quality evidence and 5 (or E) for evidence with effectiveness not established, as you can see in the pyramidal scheme below:

Level 1: (higher quality of evidence) – High-quality randomized trial or prospective study; testing of previously developed diagnostic criteria on consecutive patients; sensible costs and alternatives; values obtained from many studies with multiway sensitivity analyses; systematic review of Level I RCTs and Level I studies.

Level 2: Lesser quality RCT; prospective comparative study; retrospective study; untreated controls from an RCT; lesser quality prospective study; development of diagnostic criteria on consecutive patients; sensible costs and alternatives; values obtained from limited stud- ies; with multiway sensitivity analyses; systematic review of Level II studies or Level I studies with inconsistent results.

Level 3: Case-control study (therapeutic and prognostic studies); retrospective comparative study; study of nonconsecutive patients without consistently applied reference “gold” standard; analyses based on limited alternatives and costs and poor estimates; systematic review of Level III studies.

Level 4: Case series; case-control study (diagnostic studies); poor reference standard; analyses with no sensitivity analyses.

Level 5: (lower quality of evidence) – Expert opinion.

By looking at the pyramid, you can roughly distinguish what type of research gives you the highest quality of evidence and which gives you the lowest. Basically, level 1 and level 2 are filtered information – that means an author has gathered evidence from well-designed studies, with credible results, and has produced findings and conclusions appraised by renowned experts, who consider them valid and strong enough to serve researchers and scientists. Levels 3, 4 and 5 include evidence coming from unfiltered information. Because this evidence hasn’t been appraised by experts, it might be questionable, but not necessarily false or wrong.

Examples of levels of evidence

As you move up the pyramid, you will surely find higher-quality evidence. However, you will notice there is also less research available. So, if there are no resources for you available at the top, you may have to start moving down in order to find the answers you are looking for.

• Systematic Reviews: -Exhaustive summaries of all the existent literature about a certain topic. When drafting a systematic review, authors are expected to deliver a critical assessment and evaluation of all this literature rather than a simple list. Researchers that produce systematic reviews have their own criteria to locate, assemble and evaluate a body of literature.
• Meta-Analysis: Uses quantitative methods to synthesize a combination of results from independent studies. Normally, they function as an overview of clinical trials. Read more: Systematic review vs meta-analysis .
• Critically Appraised Topic: Evaluation of several research studies.
• Critically Appraised Article: Evaluation of individual research studies.
• Randomized Controlled Trial: a clinical trial in which participants or subjects (people that agree to participate in the trial) are randomly divided into groups. Placebo (control) is given to one of the groups whereas the other is treated with medication. This kind of research is key to learning about a treatment’s effectiveness.
• Cohort studies: A longitudinal study design, in which one or more samples called cohorts (individuals sharing a defining characteristic, like a disease) are exposed to an event and monitored prospectively and evaluated in predefined time intervals. They are commonly used to correlate diseases with risk factors and health outcomes.
• Case-Control Study: Selects patients with an outcome of interest (cases) and looks for an exposure factor of interest.
• Background Information/Expert Opinion: Information you can find in encyclopedias, textbooks and handbooks. This kind of evidence just serves as a good foundation for further research – or clinical practice – for it is usually too generalized.

Of course, it is recommended to use level A and/or 1 evidence for more accurate results but that doesn’t mean that all other study designs are unhelpful or useless. It all depends on your research question. Focusing once more on the healthcare and medical field, see how different study designs fit into particular questions, that are not necessarily located at the tip of the pyramid:

• Questions concerning therapy: “Which is the most efficient treatment for my patient?” >> RCT | Cohort studies | Case-Control | Case Studies
• Questions concerning diagnosis: “Which diagnose method should I use?” >> Prospective blind comparison
• Questions concerning prognosis: “How will the patient’s disease will develop over time?” >> Cohort Studies | Case Studies
• Questions concerning etiology: “What are the causes for this disease?” >> RCT | Cohort Studies | Case Studies
• Questions concerning costs: “What is the most cost-effective but safe option for my patient?” >> Economic evaluation
• Questions concerning meaning/quality of life: “What’s the quality of life of my patient going to be like?” >> Qualitative study

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Home » Evidence – Definition, Types and Example

Evidence – Definition, Types and Example

Table of Contents

Definition:

Evidence is any information or data that supports or refutes a claim, hypothesis, or argument. It is the basis for making decisions, drawing conclusions, and establishing the truth or validity of a statement.

Types of Evidence

Types of Evidence are as follows:

Empirical evidence

This type of evidence comes from direct observation or measurement, and is usually based on data collected through scientific or other systematic methods.

Expert Testimony

This is evidence provided by individuals who have specialized knowledge or expertise in a particular area, and can provide insight into the validity or reliability of a claim.

Personal Experience

This type of evidence comes from firsthand accounts of events or situations, and can be useful in providing context or a sense of perspective.

Statistical Evidence

This type of evidence involves the use of numbers and data to support a claim, and can include things like surveys, polls, and other types of quantitative analysis.

Analogical Evidence

This involves making comparisons between similar situations or cases, and can be used to draw conclusions about the validity or applicability of a claim.

Documentary Evidence

This includes written or recorded materials, such as contracts, emails, or other types of documents, that can provide support for a claim.

Circumstantial Evidence

This type of evidence involves drawing inferences based on indirect or circumstantial evidence, and can be used to support a claim when direct evidence is not available.

Examples of Evidence

Here are some examples of different types of evidence that could be used to support a claim or argument:

• A study conducted on a new drug, showing its effectiveness in treating a particular disease, based on clinical trials and medical data.
• A doctor providing testimony in court about a patient’s medical condition or injuries.
• A patient sharing their personal experience with a particular medical treatment or therapy.
• A study showing that a particular type of cancer is more common in certain demographics or geographic areas.
• Comparing the benefits of a healthy diet and exercise to maintaining a car with regular oil changes and maintenance.
• A contract showing that two parties agreed to a particular set of terms and conditions.
• The presence of a suspect’s DNA at the crime scene can be used as circumstantial evidence to suggest their involvement in the crime.

Applications of Evidence

Here are some applications of evidence:

• Law : In the legal system, evidence is used to establish facts and to prove or disprove a case. Lawyers use different types of evidence, such as witness testimony, physical evidence, and documentary evidence, to present their arguments and persuade judges and juries.
• Science : Evidence is the foundation of scientific inquiry. Scientists use evidence to support or refute hypotheses and theories, and to advance knowledge in their fields. The scientific method relies on evidence-based observations, experiments, and data analysis.
• Medicine : Evidence-based medicine (EBM) is a medical approach that emphasizes the use of scientific evidence to inform clinical decision-making. EBM relies on clinical trials, systematic reviews, and meta-analyses to determine the best treatments for patients.
• Public policy : Evidence is crucial in informing public policy decisions. Policymakers rely on research studies, evaluations, and other forms of evidence to develop and implement policies that are effective, efficient, and equitable.
• Business : Evidence-based decision-making is becoming increasingly important in the business world. Companies use data analytics, market research, and other forms of evidence to make strategic decisions, evaluate performance, and optimize operations.

Purpose of Evidence

The purpose of evidence is to support or prove a claim or argument. Evidence can take many forms, including statistics, examples, anecdotes, expert opinions, and research studies. The use of evidence is important in fields such as science, law, and journalism to ensure that claims are backed up by factual information and to make decisions based on reliable information. Evidence can also be used to challenge or question existing beliefs and assumptions, and to uncover new knowledge and insights. Overall, the purpose of evidence is to provide a foundation for understanding and decision-making that is grounded in empirical facts and data.

Characteristics of Evidence

Some Characteristics of Evidence are as follows:

• Relevance : Evidence must be relevant to the claim or argument it is intended to support. It should directly address the issue at hand and not be tangential or unrelated.
• Reliability : Evidence should come from a trustworthy and reliable source. The credibility of the source should be established, and the information should be accurate and free from bias.
• Sufficiency : Evidence should be sufficient to support the claim or argument. It should provide enough information to make a strong case, but not be overly repetitive or redundant.
• Validity : Evidence should be based on sound reasoning and logic. It should be based on established principles or theories, and should be consistent with other evidence and observations.
• Timeliness : Evidence should be current and up-to-date. It should reflect the most recent developments or research in the field.
• Accessibility : Evidence should be easily accessible to others who may want to review or evaluate it. It should be clear and easy to understand, and should be presented in a way that is appropriate for the intended audience.

Advantages of Evidence

The use of evidence has several advantages, including:

• Supports informed decision-making: Evidence-based decision-making enables individuals or organizations to make informed choices based on reliable information rather than assumptions or opinions.
• Enhances credibility: The use of evidence can enhance the credibility of claims or arguments by providing factual support.
• Promotes transparency: The use of evidence promotes transparency in decision-making processes by providing a clear and objective basis for decisions.
• Facilitates evaluation : Evidence-based decision-making enables the evaluation of the effectiveness of policies, programs, and interventions.
• Provides insights: The use of evidence can provide new insights and perspectives on complex issues, enabling individuals or organizations to approach problems from different angles.
• Enhances problem-solving : Evidence-based decision-making can help individuals or organizations to identify the root causes of problems and develop more effective solutions.

Limitations of Evidence

Some Limitations of Evidence are as follows:

• Limited availability : Evidence may not always be available or accessible, particularly in areas where research is limited or where data collection is difficult.
• Interpretation challenges: Evidence can be open to interpretation, and individuals may interpret the same evidence differently based on their biases, experiences, or values.
• Time-consuming: Gathering and evaluating evidence can be time-consuming and require significant resources, which may not always be feasible in certain contexts.
• May not apply universally : Evidence may be context-specific and may not apply universally to other situations or populations.
• Potential for bias: Even well-designed studies or research can be influenced by biases, such as selection bias, measurement bias, or publication bias.
• Ethical concerns : Evidence may raise ethical concerns, such as the use of personal data or the potential harm to research participants.

About the author

Muhammad Hassan

Researcher, Academic Writer, Web developer

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Evidence-Based Research Series-Paper 1: What Evidence-Based Research is and why is it important?

Affiliations.

• 1 Johns Hopkins Evidence-based Practice Center, Division of General Internal Medicine, Department of Medicine, Johns Hopkins University, Baltimore, MD, USA.
• 2 Digital Content Services, Operations, Elsevier Ltd., 125 London Wall, London, EC2Y 5AS, UK.
• 3 School of Nursing, McMaster University, Health Sciences Centre, Room 2J20, 1280 Main Street West, Hamilton, Ontario, Canada, L8S 4K1; Section for Evidence-Based Practice, Western Norway University of Applied Sciences, Inndalsveien 28, Bergen, P.O.Box 7030 N-5020 Bergen, Norway.
• 4 Department of Sport Science and Clinical Biomechanics, University of Southern Denmark, Campusvej 55, 5230, Odense M, Denmark; Department of Physiotherapy and Occupational Therapy, University Hospital of Copenhagen, Herlev & Gentofte, Kildegaardsvej 28, 2900, Hellerup, Denmark.
• 5 Musculoskeletal Statistics Unit, the Parker Institute, Bispebjerg and Frederiksberg Hospital, Copenhagen, Nordre Fasanvej 57, 2000, Copenhagen F, Denmark; Department of Clinical Research, Research Unit of Rheumatology, University of Southern Denmark, Odense University Hospital, Denmark.
• 6 Section for Evidence-Based Practice, Western Norway University of Applied Sciences, Inndalsveien 28, Bergen, P.O.Box 7030 N-5020 Bergen, Norway. Electronic address: [email protected].
• PMID: 32979491
• DOI: 10.1016/j.jclinepi.2020.07.020

Objectives: There is considerable actual and potential waste in research. Evidence-based research ensures worthwhile and valuable research. The aim of this series, which this article introduces, is to describe the evidence-based research approach.

Study design and setting: In this first article of a three-article series, we introduce the evidence-based research approach. Evidence-based research is the use of prior research in a systematic and transparent way to inform a new study so that it is answering questions that matter in a valid, efficient, and accessible manner.

Results: We describe evidence-based research and provide an overview of the approach of systematically and transparently using previous research before starting a new study to justify and design the new study (article #2 in series) and-on study completion-place its results in the context with what is already known (article #3 in series).

Conclusion: This series introduces evidence-based research as an approach to minimize unnecessary and irrelevant clinical health research that is unscientific, wasteful, and unethical.

Keywords: Clinical health research; Clinical trials; Evidence synthesis; Evidence-based research; Medical ethics; Research ethics; Systematic review.

Copyright © 2020 Elsevier Inc. All rights reserved.

Publication types

• Research Support, Non-U.S. Gov't
• Biomedical Research* / methods
• Biomedical Research* / organization & administration
• Clinical Trials as Topic / ethics
• Clinical Trials as Topic / methods
• Clinical Trials as Topic / organization & administration
• Ethics, Research
• Evidence-Based Medicine / methods*
• Needs Assessment
• Reproducibility of Results
• Research Design* / standards
• Research Design* / trends
• Systematic Reviews as Topic
• Treatment Outcome

Systematic Reviews

• Levels of Evidence
• Evidence Pyramid
• Joanna Briggs Institute

The evidence pyramid is often used to illustrate the development of evidence. At the base of the pyramid is animal research and laboratory studies – this is where ideas are first developed. As you progress up the pyramid the amount of information available decreases in volume, but increases in relevance to the clinical setting.

Meta Analysis  – systematic review that uses quantitative methods to synthesize and summarize the results.

Systematic Review  – summary of the medical literature that uses explicit methods to perform a comprehensive literature search and critical appraisal of individual studies and that uses appropriate st atistical techniques to combine these valid studies.

Randomized Controlled Trial – Participants are randomly allocated into an experimental group or a control group and followed over time for the variables/outcomes of interest.

Cohort Study – Involves identification of two groups (cohorts) of patients, one which received the exposure of interest, and one which did not, and following these cohorts forward for the outcome of interest.

Case Control Study – study which involves identifying patients who have the outcome of interest (cases) and patients without the same outcome (controls), and looking back to see if they had the exposure of interest.

Case Series   – report on a series of patients with an outcome of interest. No control group is involved.

• Levels of Evidence from The Centre for Evidence-Based Medicine
• The JBI Model of Evidence Based Healthcare
• How to Use the Evidence: Assessment and Application of Scientific Evidence From the National Health and Medical Research Council (NHMRC) of Australia. Book must be downloaded; not available to read online.

When searching for evidence to answer clinical questions, aim to identify the highest level of available evidence. Evidence hierarchies can help you strategically identify which resources to use for finding evidence, as well as which search results are most likely to be "best".                                             

Image source: Evidence-Based Practice: Study Design from Duke University Medical Center Library & Archives. This work is licensed under a Creativ e Commons Attribution-ShareAlike 4.0 International License .

The hierarchy of evidence (also known as the evidence-based pyramid) is depicted as a triangular representation of the levels of evidence with the strongest evidence at the top which progresses down through evidence with decreasing strength. At the top of the pyramid are research syntheses, such as Meta-Analyses and Systematic Reviews, the strongest forms of evidence. Below research syntheses are primary research studies progressing from experimental studies, such as Randomized Controlled Trials, to observational studies, such as Cohort Studies, Case-Control Studies, Cross-Sectional Studies, Case Series, and Case Reports. Non-Human Animal Studies and Laboratory Studies occupy the lowest level of evidence at the base of the pyramid.

• Finding Evidence-Based Answers to Clinical Questions – Quickly & Effectively A tip sheet from the health sciences librarians at UC Davis Libraries to help you get started with selecting resources for finding evidence, based on type of question.
• << Previous: What is a Systematic Review?
• Next: Locating Systematic Reviews >>
• Getting Started
• What is a Systematic Review?
• Locating Systematic Reviews
• Searching Systematically
• Developing Answerable Questions
• Identifying Synonyms & Related Terms
• Using Truncation and Wildcards
• Identifying Search Limits/Exclusion Criteria
• Keyword vs. Subject Searching
• Where to Search
• Search Filters
• Sensitivity vs. Precision
• Core Databases
• Other Databases
• Clinical Trial Registries
• Conference Presentations
• Databases Indexing Grey Literature
• Web Searching
• Handsearching
• Citation Indexes
• Documenting the Search Process
• Managing your Review

Research Support

• Last Updated: Apr 8, 2024 3:33 PM
• URL: https://guides.library.ucdavis.edu/systematic-reviews

• What is the best evidence and how to find it

Why is research evidence better than expert opinion alone?

In a broad sense, research evidence can be any systematic observation in order to establish facts and reach conclusions. Anything not fulfilling this definition is typically classified as “expert opinion”, the basis of which includes experience with patients, an understanding of biology, knowledge of pre-clinical research, as well as of the results of studies. Using expert opinion as the only basis to make decisions has proved problematic because in practice doctors often introduce new treatments too quickly before they have been shown to work, or they are too slow to introduce proven treatments.

However, clinical experience is key to interpret and apply research evidence into practice, and to formulate recommendations, for instance in the context of clinical guidelines. In other words, research evidence is necessary but not sufficient to make good health decisions.

Which studies are more reliable?

Not all evidence is equally reliable.

Any study design, qualitative or quantitative, where data is collected from individuals or groups of people is usually called a primary study. There are many types of primary study designs, but for each type of health question there is one that provides more reliable information.

For treatment decisions, there is consensus that the most reliable primary study is the randomised controlled trial (RCT). In this type of study, patients are randomly assigned to have either the treatment being tested or a comparison treatment (sometimes called the control treatment). Random really means random. The decision to put someone into one group or another is made like tossing a coin: heads they go into one group, tails they go into the other.

The control treatment might be a different type of treatment or a dummy treatment that shouldn't have any effect (a placebo). Researchers then compare the effects of the different treatments.

Large randomised trials are expensive and take time. In addition sometimes it may be unethical to undertake a study in which some people were randomly assigned not to have a treatment. For example, it wouldn't be right to give oxygen to some children having an asthma attack and not give it to others. In cases like this, other primary study designs may be the best choice.

Laboratory studies are another type of study. Newspapers often have stories of studies showing how a drug cured cancer in mice. But just because a treatment works for animals in laboratory experiments, this doesn't mean it will work for humans. In fact, most drugs that have been shown to cure cancer in mice do not work for people.

Very rarely we cannot base our health decisions on the results of studies. Sometimes the research hasn't been done because doctors are used to treating a condition in a way that seems to work. This is often true of treatments for broken bones and operations. But just because there's no research for a treatment doesn't mean it doesn't work. It just means that no one can say for sure.

Why we shouldn’t read studies

An enormous amount of effort is required to be able to identify and summarise everything we know with regard to any given health intervention. The amount of data has soared dramatically. A conservative estimation is there are more than 35,000 medical journals and almost 20 million research articles published every year. On the other hand, up to half of existing data might be unpublished.

How can anyone keep up with all this? And how can you tell if the research is good or not? Each primary study is only one piece of a jigsaw that may take years to finish. Rarely does any one piece of research answer either a doctor's, or a patient's questions.

Even though reading large numbers of studies is impractical, high-quality primary studies, especially RCTs, constitute the foundations of what we know, and they are the best way of advancing the knowledge. Any effort to support or promote the conduct of sound, transparent, and independent trials that are fully and clearly published is worth endorsing. A prominent project in this regard is the All trials initiative.

Why we should read systematic reviews

Most of the time a single study doesn't tell us enough. The best answers are found by combining the results of many studies.

A systematic review is a type of research that looks at the results from all of the good-quality studies. It puts together the results of these individual studies into one summary. This gives an estimate of a treatment's risks and benefits. Sometimes these reviews include a statistical analysis, called a meta-analysis , which combines the results of several studies to give a treatment effect.

Systematic reviews are increasingly being used for decision making because they reduce the probability of being misled by looking at one piece of the jigsaw. By being systematic they are also more transparent, and have become the gold standard approach to synthesise the ever-expanding and conflicting biomedical literature.

Systematic reviews are not fool proof. Their findings are only as good as the studies that they include and the methods they employ. But the best reviews clearly state whether the studies they include are good quality or not.

Three reasons why we shouldn’t read (most) systematic reviews

Firstly, systematic reviews have proliferated over time. From 11 per day in 2010, they skyrocketed up to 40 per day or more in 2015.[1][2] Some have described this production as having reached epidemic proportions where the large majority of produced systematic reviews and meta-analyses are unnecessary, misleading, and/or conflicted.[3][4] So, finding more than one systematic review for a question is the rule more than the exception, and it is not unusual to find several dozen for the hottest questions.

Second, most systematic reviews address a narrow question. It is difficult to put them in the context of all of the available alternatives for an individual case. Reading multiple reviews to assess all of the alternatives is impractical, even more if we consider they are typically difficult to read for the average clinician, who will need to solve several questions each day.[5]

Third, systematic reviews do not tell you what to do, or what is advisable for a given patient or situation. Indeed, good systematic reviews explicitly avoid making recommendations.

So, even though systematic reviews play a key role in any evidence-based decision-making process, most of them are low-quality or outdated, and they rarely provide all the information needed to make decisions in the real world.

How to find the best available evidence?

Considering the massive amount of information available, we can quickly discard periodically reviewing our favourite journals as a means of sourcing the best available evidence.

The traditional approach to search for evidence has been using major databases, such as PubMed  or EMBASE . These constitute comprehensive sources including millions of relevant, but also irrelevant articles. Even though in the past they were the preferred approach to searching for evidence, information overload has made them impractical, and most clinicians would fail to find the best available evidence in this way, however hard they tried.

Another popular approach is simply searching in Google. Unfortunately, because of its lack of transparency, Google is not a reliable way to filter current best evidence from unsubstantiated or non-scientifically supervised sources.[6]

Three alternatives to access the best evidence

Alternative 1 - Pick the best systematic review Mastering the art of identifying, appraising, and applying high-quality systematic reviews into practice can be very rewarding. It is not easy, but once mastered it gives a view of the bigger picture: of what is known, and what is not known.

The best single source of highest-quality systematic reviews is produced by an international organisation called the Cochrane Collaboration, named after a well-known researcher.[4] They can be accessed at The Cochrane Library .

Unfortunately, Cochrane reviews do not cover all of the existing questions and they are not always up to date. Also, there might be non-Cochrane reviews out-performing Cochrane reviews.

There are many resources that facilitate access to systematic reviews (and other resources), such as Trip database , PubMed Health , ACCESSSS , or Epistemonikos (the Cochrane Collaboration maintains a comprehensive list of these resources).

Epistemonikos database is innovative both in simultaneously searching multiple resources and in indexing and interlinking relevant evidence. For example, Epistemonikos connects systematic reviews and their included studies, and thus allows clustering of systematic reviews based on the primary studies they have in common. Epistemonikos is also unique in offering an appreciable multilingual user interface, multilingual search, and translation of abstracts in more than nine languages.[6] This database includes several tools to compare systematic reviews, including the matrix of evidence, a dynamic table showing all of the systematic reviews, and the primary studies included in those reviews.

Additionally, Epistemonikos partnered with Cochrane, and during 2017 a combined search in both the Cochrane Library and Epistemonikos was released.

Alternative 2 - Read trustworthy guidelines Although systematic reviews can provide a synthesis of the benefits and harms of the interventions, they do not integrate these factors with patients’ values and preferences or resource considerations to provide a suggested course of action. Also, to fully address the questions, clinicians would need to integrate the information of several systematic reviews covering all the relevant alternatives and outcomes. Most clinicians will likely prefer guidance rather than interpreting systematic reviews themselves.

Trustworthy guidelines, especially if developed with high standards, such as the Grading of Recommendations, Assessment, Development, and Evaluation ( GRADE ) approach, offer systematic and transparent guidance in moving from evidence to recommendations.[7]

Many online guideline websites promote themselves as “evidence based”, but few have explicit links to research findings.[8] If they don’t have in-line references to relevant research findings, dismiss them. If they have, you can judge the strength of the commitment to evidence to support inference, checking whether statements are based on high-quality versus low-quality evidence using alternative 1 explained above.

Unfortunately, most guidelines have serious limitations or are outdated.[9][10] The exercise of locating and appraising the best guideline is time consuming. This is particularly challenging for generalists addressing questions from different conditions or diseases.

Alternative 3 - Use point-of-care tools Point-of-care tools, such as BMJ Best Practice, have been developed as a response to the genuine need to summarise the ever-expanding biomedical literature on an ever-increasing number of alternatives in order to make evidence-based decisions. In this competitive market, the more successful products have been those delivering innovative, user-friendly interfaces that improve the retrieval, synthesis, organisation, and application of evidence-based content in many different areas of clinical practice.

However, the same impossibility in catching up with new evidence without compromising quality that affects guidelines also affects point-of-care tools. Clinicians should become familiar with the point-of-care information resource they want or can access, and examine the in-line references to relevant research findings. Clinicians can easily judge the strength of the commitment to evidence checking whether statements are based on high-quality versus low-quality evidence using alternative 1 explained above. Comprehensiveness, use of GRADE approach, and independence are other characteristics to bear in mind when selecting among point-of-care information summaries.

A comprehensive list of these resources can be found in a study by Kwag et al .

Finding the best available evidence is more challenging than it was in the dawn of the evidence-based movement, and the main cause is the exponential growth of evidence-based information, in any of the flavours described above.

However, with a little bit of patience and practice, the busy clinician will discover evidence-based practice is far easier than it was 5 or 10 years ago. We are entering a stage where information is flowing between the different systems, technology is being harnessed for good, and the different players are starting to generate alliances.

The early adopters will surely enjoy the first experiments of living systematic reviews (high-quality, up-to-date online summaries of health research that are updated as new research becomes available), living guidelines, and rapid reviews tied to rapid recommendations, just to mention a few. [13][14][15]

It is unlikely that the picture of countless low-quality studies and reviews will change in the foreseeable future. However, it would not be a surprise if, in 3 to 5 years, separating the wheat from the chaff becomes trivial. Maybe the promise of evidence-based medicine of more effective, safer medical intervention resulting in better health outcomes for patients could be fulfilled.

Author: Gabriel Rada

Competing interests: Gabriel Rada is the co-founder and chairman of Epistemonikos database, part of the team that founded and maintains PDQ-Evidence, and an editor of the Cochrane Collaboration.

 Related Blogs

  Living Systematic Reviews: towards real-time evidence for health-care decision making

• Bastian H, Glasziou P, Chalmers I. Seventy-five trials and eleven systematic reviews a day: how will we ever keep up? PLoS Med. 2010 Sep 21;7(9):e1000326. doi: 10.1371/journal.pmed.1000326
• Epistemonikos database [filter= systematic review; year=2015]. A Free, Relational, Collaborative, Multilingual Database of Health Evidence. https://www.epistemonikos.org/en/search?&q=*&classification=systematic-review&year_start=2015&year_end=2015&fl=14542 Accessed 5 Jan 2017.
• Ioannidis JP. The Mass Production of Redundant, Misleading, and Conflicted Systematic Reviews and Meta-analyses. Milbank Q. 2016 Sep;94(3):485-514. doi: 10.1111/1468-0009.12210.
• Page MJ, Shamseer L, Altman DG, et al. Epidemiology and reporting characteristics of systematic reviews of biomedical research: a cross-sectional study. PLoS Med. 2016;13(5):e1002028.
• Del Fiol G, Workman TE, Gorman PN. Clinical questions raised by clinicians at the point of care: a systematic review. JAMA Intern Med. 2014 May;174(5):710-8. doi: 10.1001/jamainternmed.2014.368.
• Agoritsas T, Vandvik P, Neumann I, Rochwerg B, Jaeschke R, Hayward R, et al. Chapter 5: finding current best evidence. In: Users' guides to the medical literature: a manual for evidence-based clinical practice. Chicago: MacGraw-Hill, 2014.
• Guyatt GH, Oxman AD, Vist GE, et al. GRADE: An emerging consensus on rating quality of evidence and strength of recommendations. BMJ. 2008;336(7650):924-926. doi: 10.1136/bmj.39489.470347
• Neumann I, Santesso N, Akl EA, Rind DM, Vandvik PO, Alonso-Coello P, Agoritsas T, Mustafa RA, Alexander PE, Schünemann H, Guyatt GH. A guide for health professionals to interpret and use recommendations in guidelines developed with the GRADE approach. J Clin Epidemiol. 2016 Apr;72:45-55. doi: 10.1016/j.jclinepi.2015.11.017
• Alonso-Coello P, Irfan A, Solà I, Gich I, Delgado-Noguera M, Rigau D, Tort S, Bonfill X, Burgers J, Schunemann H. The quality of clinical practice guidelines over the last two decades: a systematic review of guideline appraisal studies. Qual Saf Health Care. 2010 Dec;19(6):e58. doi: 10.1136/qshc.2010.042077
• Martínez García L, Sanabria AJ, García Alvarez E, Trujillo-Martín MM, Etxeandia-Ikobaltzeta I, Kotzeva A, Rigau D, Louro-González A, Barajas-Nava L, Díaz Del Campo P, Estrada MD, Solà I, Gracia J, Salcedo-Fernandez F, Lawson J, Haynes RB, Alonso-Coello P; Updating Guidelines Working Group. The validity of recommendations from clinical guidelines: a survival analysis. CMAJ. 2014 Nov 4;186(16):1211-9. doi: 10.1503/cmaj.140547
• Kwag KH, González-Lorenzo M, Banzi R, Bonovas S, Moja L. Providing Doctors With High-Quality Information: An Updated Evaluation of Web-Based Point-of-Care Information Summaries. J Med Internet Res. 2016 Jan 19;18(1):e15. doi: 10.2196/jmir.5234
• Banzi R, Cinquini M, Liberati A, Moschetti I, Pecoraro V, Tagliabue L, Moja L. Speed of updating online evidence based point of care summaries: prospective cohort analysis. BMJ. 2011 Sep 23;343:d5856. doi: 10.1136/bmj.d5856
• Elliott JH, Turner T, Clavisi O, Thomas J, Higgins JP, Mavergames C, Gruen RL. Living systematic reviews: an emerging opportunity to narrow the evidence-practice gap. PLoS Med. 2014 Feb 18;11(2):e1001603. doi: 10.1371/journal.pmed.1001603
• Vandvik PO, Brandt L, Alonso-Coello P, Treweek S, Akl EA, Kristiansen A, Fog-Heen A, Agoritsas T, Montori VM, Guyatt G. Creating clinical practice guidelines we can trust, use, and share: a new era is imminent. Chest. 2013 Aug;144(2):381-9. doi: 10.1378/chest.13-0746
• Vandvik PO, Otto CM, Siemieniuk RA, Bagur R, Guyatt GH, Lytvyn L, Whitlock R, Vartdal T, Brieger D, Aertgeerts B, Price S, Foroutan F, Shapiro M, Mertz R, Spencer FA. Transcatheter or surgical aortic valve replacement for patients with severe, symptomatic, aortic stenosis at low to intermediate surgical risk: a clinical practice guideline. BMJ. 2016 Sep 28;354:i5085. doi: 10.1136/bmj.i5085

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Navigating Scientific Evidence: Types and Definitions

This article provides an insightful exploration into the varied landscape of scientific evidence. It delves into its principles, utility, and its role in hypothesis testing.

With perspectives from philosophers and scientists, it presents a comprehensive understanding of evidence statements and hypotheses.

The article underscores the significance of testing scientific ideas and the influence of special interest groups on research. It also highlights the integral role of the scientific community in the progression of knowledge.

Key Takeaways

• Different background beliefs can lead to different conclusions drawn from the same scientific evidence.
• Scientific evidence plays a central role in Karl R. Popper’s theory of the scientific method.
• Falsifiability is an important concept in determining the utility of scientific evidence.
• Scientists actively seek evidence to test their ideas, even if it requires significant effort.

Understanding the Principles of Scientific Evidence

The principles of scientific evidence, which form the bedrock of empirical research, entail a meticulous understanding of how observations relate with hypotheses and how these relationships are interpreted, often influenced by underlying assumptions or beliefs. This scientific definition of evidence underscores the inherent objectivity and rigorousness of the scientific method.

Evidence serves as a tangible manifestation of abstract theories, often swaying the pendulum of scientific discourse. It is through the judicious application of these principles that scholars can transform raw data into reliable knowledge.

Evidence, then, is not merely an accumulation of facts but a dynamic tool employed to challenge, validate, or refine existing hypotheses. This intricate process underscores the importance of both the acquisition and interpretation of scientific evidence.

Distinguishing Philosophical and Scientific Views on Evidence

In the realm of academia, distinguishing between philosophical and scientific views on evidence presents a fascinating discourse, highlighting the divergences and synergies in interpretation, methodology, and epistemological underpinnings.

Philosophically, evidence is evaluated through abstract, conceptual analysis while scientific evidence is more empirically grounded. The scientific evidence definition rests on the premise that it is concrete, measurable, reproducible, and consistent with theoretical expectations. Philosophy, on the contrary, explores the nuances of evidence, often delving into the subjective realm.

Nevertheless, both views contribute to an all-encompassing understanding of evidence, underlining its multifaceted nature.

Ultimately, the ongoing dialogue between philosophical and scientific perspectives deepens our comprehension of evidence and its pivotal role in advancing knowledge.

Exploring Different Concepts of Evidence

Several concepts of evidence provide a diverse range of perspectives that aid in comprehending the multifaceted nature of scientific inquiry. These concepts, categorised under different types of scientific evidence, play an integral role in the formulation and validation of theories.

For instance, subjective evidence is based on individual perceptions, while veridical evidence is grounded in observable facts. Importantly, potential evidence represents unverified data that may, upon validation, solidify into veridical evidence.

Scientists often seek veridical evidence due to its factual basis but also employ other evidence types to develop comprehensive analyses. The utilisation of these varied concepts contributes to a more robust and nuanced understanding of phenomena, emphasising the complexity and richness of scientific investigation.

The Crucial Role of Hypothesis Testing in Science

Undeniably, hypothesis testing serves as a fundamental pillar in scientific research, facilitating rigorous scrutiny and validation of theories. It enables researchers to draw conclusions from scientific evidence, thus propelling the growth of knowledge. Through hypothesis testing, theories are either supported or rejected based on the strength and credibility of the evidence.

This process underscores the integral role of scientific evidence in research. Without hypothesis testing, scientific research would lack objectivity, reliability, and credibility. Therefore, the importance of hypothesis testing in validating scientific evidence cannot be overstated.

The Relationship Between Evidence and Ideas in Scientific Practice

Remarkably, the relationship between evidence and ideas in scientific practice serves as the cornerstone of any meaningful scientific discovery, with evidence acting as the litmus test for the validity and reliability of ideas. This relationship is critical, as it provides a structured pathway for scientists to formulate, test, and validate their hypotheses.

To illustrate this, consider these examples of scientific evidence:

• Fossils serving as evidence for evolution.
• The Doppler shift in light from distant galaxies as evidence for the Big Bang theory.
• Genetic mutations as evidence for natural selection.
• Climate data as evidence for global warming.

These examples highlight the pivotal role evidence plays in shaping and refining scientific theories, ultimately driving scientific progress.

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Frequently asked questions, how has the concept of scientific evidence evolved over the centuries.

The concept of scientific evidence has transformed over centuries, influenced by advancements in technology and methodology. It has evolved from anecdotal observations to rigorous, replicable experiments, quantitative analysis, and sophisticated statistical inference.

What Is the Hierarchy of Scientific Evidence?

The hierarchy of scientific evidence is a system used to rank the reliability of different types of research findings. At the top are systematic reviews and meta-analyses, which synthesise data from multiple studies, followed by randomised controlled trials, cohort studies, case-control studies, case series/reports, and expert opinion. This ranking helps guide decision-making by indicating which evidence is most likely to be accurate.

What Are the Ethical Considerations When Gathering and Interpreting Scientific Evidence?

Ethical considerations in gathering and interpreting scientific evidence include ensuring accuracy, objectivity, and integrity. This involves unbiased data collection, transparent methodologies, thorough data analysis, and honest reporting of findings, while respecting confidentiality and privacy norms.

How Can Non-Scientists Evaluate the Reliability of Scientific Evidence Presented in the Media?

Non-scientists can evaluate the reliability of scientific evidence presented in the media by scrutinising the source, checking for peer reviews, looking at the methodology, and cross-verifying the information with other credible sources.

How Does the Process of Peer Review Contribute to the Validation of Scientific Evidence?

The peer review process validates scientific evidence by ensuring it undergoes rigorous scrutiny by other experts in the field. This process verifies the integrity, accuracy, and reliability of the research before it is published.

How Does Cultural or Societal Context Influence the Interpretation of Scientific Evidence?

Cultural or societal context greatly influences the interpretation of scientific evidence, as it can shape individual perception and understanding. Bias, traditions, and societal norms can impact how scientific data is evaluated and accepted.

How Does Scientific Evidence affect Scientific Content Creation?

Scientific evidence fundamentally shapes scientific content creation by providing the empirical foundation upon which accurate, reliable, and meaningful content is built. In the process of content creation, whether it be for academic journals, educational materials, or public science communication, the inclusion of scientific evidence ensures that information is grounded in verified research and observations. This reliance on evidence allows for the formulation of hypotheses, theories, and models that accurately reflect the natural world.

In conclusion, the interpretation of scientific evidence is guided by principles, philosophical views, and systematic testing of hypotheses.

The role of the scientific community in progressing knowledge is irrefutable.

However, the influence of special interest groups can potentially skew results.

A clear understanding of differing concepts of evidence and the relationship between evidence and ideas is vital in the pursuit and application of scientific knowledge.

Discover the ScioWire research newsfeed: summarised scientific knowledge ready to digest.

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Finding Types of Research

• Evidence-Based Research

On This Guide

About this guide, understand evidence-based practice, identify research study types.

• Quantitative Studies
• Qualitative Studies
• Meta-Analysis
• Systematic Reviews
• Randomized Controlled Trials
• Observational Studies
• Literature Reviews
• Finding Research Tools This link opens in a new window

Throughout your schooling, you may need to find different types of evidence and research to support your course work. This guide provides a high-level overview of evidence-based practice as well as the different types of research and study designs. Each page of this guide offers an overview and search tips for finding articles that fit that study design.

Note! If you need help finding a specific type of study, visit the  Get Research Help guide  to contact the librarians.

What is Evidence-Based Practice?

One of the requirements for your coursework is to find articles that support evidence-based practice. But what exactly is evidence-based practice? Evidence-based practice is a method that uses relevant and current evidence to plan, implement and evaluate patient care. This definition is included in the video below, which explains all the steps of evidence-based practice in greater detail.

• Video - Evidence-based practice: What it is and what it is not. Medcom (Producer), & Cobb, D. (Director). (2017). Evidence-based practice: What it is and what it is not [Streaming Video]. United States of America: Producer. Retrieved from Alexander Street Press Nursing Education Collection

Quantitative and Qualitative Studies

Research is broken down into two different types: quantitative and qualitative. Quantitative studies are all about measurement. They will report statistics of things that can be physically measured like blood pressure, weight and oxygen saturation. Qualitative studies, on the other hand, are about people's experiences and how they feel about something. This type of information cannot be measured using statistics. Both of these types of studies report original research and are considered single studies. Watch the video below for more information.

Study Designs

Some research study types that you will encounter include:

• Case-Control Studies
• Cohort Studies
• Cross-Sectional Studies

Studies that Synthesize Other Studies

Sometimes, a research study will look at the results of many studies and look for trends and draw conclusions. These types of studies include:

• Meta Analyses

Tip! How do you determine the research article's study type or level of evidence? First, look at the article abstract. Most of the time the abstract will have a methodology section, which should tell you what type of study design the researchers are using. If it is not in the abstract, look for the methodology section of the article. It should tell you all about what type of study the researcher is doing and the steps they used to carry out the study.

Read the book below to learn how to read a clinical paper, including the types of study designs you will encounter.

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Search catalog, evidence-based practice: types of evidence.

• Introduction
• Finding Evidence (PICO)
• Types of Evidence
• Appraising Evidence
• EBP Resources
• Write a Systematic Review
• Evidence-Based Dentistry
• Evidence-Based Nursing
• Evidence-Based SPL and Audiology
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Types of Research

Once you have your focused question, it's time to decide on the type of evidence you need to answer it. Understanding the types of research will help guide you to proper evidence that will support your question.

Evidence Based Pyramid

Hierarchy of evidence and research designs.

As you move up the pyramid, the study designs are more rigorous and are less biased.

What type of study should you use?

Question definitions:.

Intervention/Therapy: Questions addressing the treatment of an illness or disability.

Etiology: Questions addressing the causes or origins of disease (i.e., factors that produce or predispose toward a certain disease or disorder).

Diagnosis: Questions addressing the act or process of identifying or determining the nature and cause of a disease or injury through evaluation.

Prognosis/Prediction: Questions addressing the prediction of the course of a disease.

The type of question you have will often lead you to the type of research that will best answer the question:

Intervention/Prevention:   RCT > Cohort Study > Case Control > Case Series

Therapy:   RCT > Cohort > Case Control > Case Series

Prognosis/Prediction:   Cohort Study > Case Control > Case Series

Diagnosis/Diagnostic:   Prospective, blind comparison to Gold Standard

Etiology:   RCT > Cohort Study > Case Control > Case Series

Definitions

Cebm study design tree.

The type of study can generally be worked at by looking at three issues:

Q1. What was the aim of the study?

• To simply describe a population (PO questions) descriptive
• To quantify the relationship between factors (PICO questions) analytic.

Q2. If analytic, was the intervention randomly allocated?

• Yes? RCT
• No? Observational study

For observational study the main types will then depend on the timing of the measurement of outcome, so our third question is:

Q3. When were the outcomes determined?

• Some time after the exposure or intervention? cohort study (‘prospective study’)
• At the same time as the exposure or intervention? cross sectional study or survey
• Before the exposure was determined? case-control study (‘retrospective study’ based on recall of the exposure)

from Centre for Evidence-Based Medicine https://www.cebm.net/2014/04/study-designs/

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• Next: Appraising Evidence >>
• Last Updated: Jan 16, 2024 2:02 PM
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12.1 Introducing Research and Research Evidence

Learning outcomes.

By the end of this section, you will be able to:

• Articulate how research evidence and sources are key rhetorical concepts in presenting a position or an argument.
• Locate and distinguish between primary and secondary research materials.
• Implement methods and technologies commonly used for research and communication within various fields.

The writing tasks for this chapter and the next two chapters are based on argumentative research. However, not all researched evidence (data) is presented in the same genre. You may need to gather evidence for a poster, a performance, a story, an art exhibit, or even an architectural design. Although the genre may vary, you usually will be required to present a perspective , or viewpoint, about a debatable issue and persuade readers to support the “validity of your viewpoint,” as discussed in Position Argument: Practicing the Art of Rhetoric . Remember, too, that a debatable issue is one that has more than a single perspective and is subject to disagreement.

The Research Process

Although individual research processes are rhetorically situated, they share some common aspects:

• Interest. The researcher has a genuine interest in the topic. It may be difficult to fake curiosity, but it is possible to develop it. Some academic assignments will allow you to pursue issues that are personally important to you; others will require you to dive into the research first and generate interest as you go.
• Questions. The researcher asks questions. At first, these questions are general. However, as researchers gain more knowledge, the questions become more sharply focused. No matter what your research assignment is, begin by articulating questions, find out where the answers lead, and then ask still more questions.
• Answers. The researcher seeks answers from people as well as from print and other media. Research projects profit when you ask knowledgeable people, such as librarians and other professionals, to help you answer questions or point you in directions to find answers. Information about research is covered more extensively in Research Process: Accessing and Recording Information and Annotated Bibliography: Gathering, Evaluating, and Documenting Sources .
• Field research. The researcher conducts field research. Field research allows researchers not only to ask questions of experts but also to observe and experience directly. It allows researchers to generate original data. No matter how much other people tell you, your knowledge increases through personal observations. In some subject areas, field research is as important as library or database research. This information is covered more extensively in Research Process: Accessing and Recording Information .
• Examination of texts. The researcher examines texts. Consulting a broad range of texts—such as magazines, brochures, newspapers, archives, blogs, videos, documentaries, or peer-reviewed journals—is crucial in academic research.
• Evaluation of sources. The researcher evaluates sources. As your research progresses, you will double-check information to find out whether it is confirmed by more than one source. In informal research, researchers evaluate sources to ensure that the final decision is satisfactory. Similarly, in academic research, researchers evaluate sources to ensure that the final product is accurate and convincing. Previewed here, this information is covered more extensively in Research Process: Accessing and Recording Information .
• Writing. The researcher writes. The writing during the research process can take a range of forms: from notes during library, database, or field work; to journal reflections on the research process; to drafts of the final product. In practical research, writing helps researchers find, remember, and explore information. In academic research, writing is even more important because the results must be reported accurately and thoroughly.
• Testing and Experimentation. The researcher tests and experiments. Because opinions vary on debatable topics and because few research topics have correct or incorrect answers, it is important to test and conduct experiments on possible hypotheses or solutions.
• Synthesis. The researcher synthesizes. By combining information from various sources, researchers support claims or arrive at new conclusions. When synthesizing, researchers connect evidence and ideas, both original and borrowed. Accumulating, sorting, and synthesizing information enables researchers to consider what evidence to use in support of a thesis and in what ways.
• Presentation. The researcher presents findings in an interesting, focused, and well-documented product.

Types of Research Evidence

Research evidence usually consists of data, which comes from borrowed information that you use to develop your thesis and support your organizational structure and reasoning. This evidence can take a range of forms, depending on the type of research conducted, the audience, and the genre for reporting the research.

Primary Research Sources

Although precise definitions vary somewhat by discipline, primary data sources are generally defined as firsthand accounts, such as texts or other materials produced by someone drawing from direct experience or observation. Primary source documents include, but are not limited to, personal narratives and diaries; eyewitness accounts; interviews; original documents such as treaties, official certificates, and government documents detailing laws or acts; speeches; newspaper coverage of events at the time they occurred; observations; and experiments. Primary source data is, in other words, original and in some way conducted or collected primarily by the researcher. The Research Process: Where to Look for Existing Sources and Compiling Sources for an Annotated Bibliography contain more information on both primary and secondary sources.

Secondary Research Sources

Secondary sources , on the other hand, are considered at least one step removed from the experience. That is, they rely on sources other than direct observation or firsthand experience. Secondary sources include, but are not limited to, most books, articles online or in databases, and textbooks (which are sometimes classified as tertiary sources because, like encyclopedias and other reference works, their primary purpose might be to summarize or otherwise condense information). Secondary sources regularly cite and build upon primary sources to provide perspective and analysis. Effective use of researched evidence usually includes both primary and secondary sources. Works of history, for example, draw on a large range of primary and secondary sources, citing, analyzing, and synthesizing information to present as many perspectives of a past event in as rich and nuanced a way as possible.

It is important to note that the distinction between primary and secondary sources depends in part on their use: that is, the same document can be both a primary source and a secondary source. For example, if Scholar X wrote a biography about Artist Y, the biography would be a secondary source about the artist and, at the same time, a primary source about the scholar.

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Evidence-Based Practice: Types of Research

• What is EBP?
• PICO and SPIDER
• More Resources
• Types of Research
• Levels of Evidence
• How to Search for Evidence
• Where to Search for Evidence
• Literature Reviews / Systematic Reviews
• Clinical Colleagues as a Source of Evidence
• What is Being Appraised?
• Critical Appraisal Tools
• Step 4: Apply
• Step 5: Assess/Audit
• EBP Resources by Discipline

Quantitative Research

• Case report or case series - a report on one or more individual patients.  There is no "control group" so this study type is considered to have low statistical validity
• Case control study - looks at patients with a particular outcome (cases) as well as "control patients" who don't have that outcome. Is useful in aetiology (causation) research but prone to causation error
• Cohort study – identifies and follows two groups (cohorts) of patients, one having received the intervention being studied, and one having not. Useful in both aetiology and prognosis research. Because the groups are not randomised, they may differ in ways other than in the variable being studied
• Randomised Controlled Trial (RCT) - a clinical trial in which participants are randomly allocated to a test treatment and a control. This is considered the “gold standard” in testing the efficacy of an intervention. RCTs incorporate the techniques of  randomisation and blinding , which reduce the potential for bias and provide good evidence for cause and effect.

Qualitative Research

• Documents - the study of documentary accounts of events, such as minutes of meetings
• Passive observation - the systematic watching and recording of behaviour  
• Participant observation – here, the researcher occupies a role or part in the setting, in addition to observing
• In-depth interview - a face-to-face conversation to explore issues or topics in detail
• Focus group - a method of group interview which explicitly includes and makes use of the group interaction to generate data.

Mixed Methods

A research study does not have to be exclusively quantitative or qualitative. Many studies will use a combination of both types of research.

In the Dictionary of Statistics and Methodology , Mixed-Method Research is defined as:

"Inquiry that combines two or more methods. This particular term usually refers to mixing that crosses the quantitative-qualitative boundary. However, that boundary is not necessarily the most difficult one to cross. For example, mixing surveys and experiments (both quantitative methods) may require more effort for many researchers than combining surveys and focus groups (the first quantitative and the second qualitative)."

Mixed method research. (2005). In P. W. Vogt (Ed.),  Dictionary of statistics & methodology (3rd ed.). http://dx.doi.org.ezproxy.csu.edu.au/10.4135/9781412983907.n1190

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What type of evidence should I use?

There are two types of evidence.

First hand research is research you have conducted yourself such as interviews, experiments, surveys, or personal experience and anecdotes.

Second hand research is research you are getting from various texts that has been supplied and compiled by others such as books, periodicals, and Web sites.

Regardless of what type of sources you use, they must be credible. In other words, your sources must be reliable, accurate, and trustworthy.

How do I know if a source is credible?

You can ask the following questions to determine if a source is credible.

Who is the author? Credible sources are written by authors respected in their fields of study. Responsible, credible authors will cite their sources so that you can check the accuracy of and support for what they've written. (This is also a good way to find more sources for your own research.)

How recent is the source? The choice to seek recent sources depends on your topic. While sources on the American Civil War may be decades old and still contain accurate information, sources on information technologies, or other areas that are experiencing rapid changes, need to be much more current.

What is the author's purpose? When deciding which sources to use, you should take the purpose or point of view of the author into consideration. Is the author presenting a neutral, objective view of a topic? Or is the author advocating one specific view of a topic? Who is funding the research or writing of this source? A source written from a particular point of view may be credible; however, you need to be careful that your sources don't limit your coverage of a topic to one side of a debate.

What type of sources does your audience value? If you are writing for a professional or academic audience, they may value peer-reviewed journals as the most credible sources of information. If you are writing for a group of residents in your hometown, they might be more comfortable with mainstream sources, such as Time or Newsweek . A younger audience may be more accepting of information found on the Internet than an older audience might be.

Be especially careful when evaluating Internet sources! Never use Web sites where an author cannot be determined, unless the site is associated with a reputable institution such as a respected university, a credible media outlet, government program or department, or well-known non-governmental organizations. Beware of using sites like Wikipedia , which are collaboratively developed by users. Because anyone can add or change content, the validity of information on such sites may not meet the standards for academic research.

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• Major UK non-commercial sponsors’ efforts to reduce research waste: a mixed-methods study
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• http://orcid.org/0000-0001-9346-3319 Till Bruckner 1 , 2 ,
• Aminul Schuster 3 ,
• Belén Chavarría 4 ,
• Carolina Cruz 5 ,
• Fabiola Karely Lizárraga Illán 5 ,
• Ronak Borana 6 ,
• Tungamirai Ishe Bvute 7 ,
• Daniel Sánchez 5
• 1 UiT The Arctic University , Tromso , Norway
• 2 TranspariMED , Bristol , UK
• 3 University of Westminster , London , UK
• 4 National Autonomous University of Nicaragua-Leon , Leon , Nicaragua
• 5 Universidad de Guadalajara , Guadalajara , Mexico
• 6 XLRI , Jamshedpur , Jharkhand , India
• 7 Independent Researcher , Dublin , Ireland
• Correspondence to Dr Till Bruckner, UiT The Arctic University of Norway, Tromso, Troms, Norway; tillbruckner{at}gmail.com

https://doi.org/10.1136/bmjebm-2023-112540

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• Health Services Research
• PUBLIC HEALTH

Worldwide, a significant proportion of clinical trials end up as costly research waste because their results are never made public. 1–3 The resulting gaps in the medical evidence base harm patients and undermine public health. 4 The Declaration of Helsinki and the WHO both call for all clinical trial results to be made public. 5 6

In the wake of a 2018 UK parliamentary enquiry, non-commercial clinical trial sponsors in the UK substantially improved outcome reporting for drug trials (Clinical Trials of Investigative Medicinal Products, CTIMPs) by uploading the summary results of many CTIMPs onto the European Union Clinical Trials Register (EUCTR) which exclusively lists drug trials. However, these efforts typically did not extend to other types of trials, which are listed on other trial registries.

This study assesses the current publication status of 145 clinical trials that are not CTIMPs that were sponsored by ten major UK non-commercial sponsors and were completed or terminated in 2017, allowing a 5-year follow-up period for publication.

The lead researcher (TB) identified the 10 most prolific non-commercial sponsors of clinical trials in the UK by accessing the EU Trials Tracker on 27 October 2022, employing the number of CTIMPs run by each sponsor as a proxy indicator of overall trial activity. 7 The lead researcher then identified other clinical trials run by these sponsors on the two other trial registries commonly used by UK sponsors, ClinicalTrials.gov and ISRCTN.

The final sample consists of 145 interventional clinical trials run by ten major non-commercial UK sponsors that are registered on ClinicalTrials.gov or ISRCTN and that were completed or terminated during 2017. These trials had a combined (actual or planned) enrolment of 34 102 patients.

The study protocol defined trial outcomes published as tabular summary results on clinical trial registries, as articles published in peer-reviewed journals, or as PhD theses as ‘fully reported’. Conference abstracts, posters, presentation slides and other documents containing trial outcomes were classified as ‘grey literature’.

During the initial data extraction, 25/145 trials were identified as having tabular summary results available on ClinicalTrials.gov; these were marked as ‘reported’ prior to the literature search. The study team then used a three-step process for locating publications for the remaining 120/145 trials. The search strategy is detailed in the study protocol ( https://osf.io/rh3m9 ). The lead researcher reviewed all search results.

The lead researcher contacted all sponsors with a list of their trials asking them to verify the publication status. All sponsors responded.

The study protocol was registered on the Open Science Framework (OSF, https://osf.io/rh3m9 ). An ethics waiver was secured. This study was funded by the charity HealthSense UK. The authors have no conflicts of interest to declare. Patients or the public were not involved in the design, or conduct, or reporting, or dissemination plans of our research. The outcomes of this study are reported in line with the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guideline for cohort studies. The study protocol, dataset, literature search guide, ethics waiver and sponsors’ responses are available on GitHub ( https://github.com/TillBruckner/UKtrials ).

In total, 116/145 trials (80%) had reported results on a registry, in the academic literature, or in a PhD thesis. The outcomes of 11/145 trials (8%) had been reported in the grey literature. Results for 18/145 trials (12%) remained completely unpublished (see table 1 ).

• View inline

Number and percentage of trials that have not fully reported results

In total, 637 people were enrolled in the 18 clinical trials that remained completely unreported. Conservatively assuming an average cost per trial of £500 000, 8 the total aggregate cost of those 18 trials was £9 million.

Sponsors indicated that they still planned to make public the results of the 14/18 unreported trials. For the remaining 4/18 trials, sponsors indicated that there were no data of value to publish due to early termination of the trial (3/18 trials) or data quality issues (1/18 trials).

This study found a registry or journal publication rate of 80% for this sample of clinical trials after a follow-up period of 5–6 years. This rate is far higher than what comparable studies have found in other countries. 1 2

However, the findings cannot be generalised to all UK university-sponsored non-CTIMPs as the study sample was small and restricted to large sponsors. Also, at least one sponsor rapidly reported several results following our outreach. A key strength is that we succeeded in verifying the publication status of every clinical trial with the responsible sponsor.

The is currently no UK legal requirement to report non-CTIMP results. Nonetheless, all sponsors initiated efforts to publish missing results following our outreach. The entire budget of this project was far less than the cost of a typical clinical trial. Replicating this project on a larger scale could be a highly cost-effective way to reduce research waste.

The UK’s national #MakeItPublic strategy commits the UK Health Research Authority (HRA) to identifying and following up on all future unreported clinical trials (see table 2 ). 9 The HRA has so far failed to deliver on this promise. Full implementation of the strategy could significantly and sustainably reduce future research waste in the UK. Requiring all trial results to be made public on trial registries, as recommended by the WHO, could lower HRA monitoring costs and improve the speed, accuracy and completeness of outcome reporting. 10

Unreported clinical trials and future publication plans

Ethics statements

Patient consent for publication.

Not applicable.

• Nelson JT ,
• Puplampu-Dove Y , et al
• Franzen DL ,
• Carlisle BG ,
• Salholz-Hillel M , et al
• Rodgers F ,
• Pepperrell T ,
• Keestra S , et al
• World Medical Association
• World Health Assembly
• Goldacre B ,
• DeVito NJ ,
• Heneghan C , et al
• Harrison J ,
• Vrijens F , et al
• UK Health Research Authority
• World Health Organization

X @TranspariMED, @aminul_schuster, @BelnChavarria03

Contributors TB conceived and designed the study, wrote the protocol and drafted the manuscript. AS, BC, CC, FKLI, RB, TIB and DS extracted and analysed the data, and critically reviewed the manuscript prior to submission. All authors approved the final manuscript and agreed to be held accountable for its contents.

Funding This study was funded by HealthSense UK (none).

Competing interests None declared.

Provenance and peer review Not commissioned; externally peer reviewed.

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What the data says about abortion in the U.S.

Pew Research Center has conducted many surveys about abortion over the years, providing a lens into Americans’ views on whether the procedure should be legal, among a host of other questions.

In a  Center survey  conducted nearly a year after the Supreme Court’s June 2022 decision that  ended the constitutional right to abortion , 62% of U.S. adults said the practice should be legal in all or most cases, while 36% said it should be illegal in all or most cases. Another survey conducted a few months before the decision showed that relatively few Americans take an absolutist view on the issue .

Find answers to common questions about abortion in America, based on data from the Centers for Disease Control and Prevention (CDC) and the Guttmacher Institute, which have tracked these patterns for several decades:

How many abortions are there in the U.S. each year?

How has the number of abortions in the u.s. changed over time, what is the abortion rate among women in the u.s. how has it changed over time, what are the most common types of abortion, how many abortion providers are there in the u.s., and how has that number changed, what percentage of abortions are for women who live in a different state from the abortion provider, what are the demographics of women who have had abortions, when during pregnancy do most abortions occur, how often are there medical complications from abortion.

This compilation of data on abortion in the United States draws mainly from two sources: the Centers for Disease Control and Prevention (CDC) and the Guttmacher Institute, both of which have regularly compiled national abortion data for approximately half a century, and which collect their data in different ways.

The CDC data that is highlighted in this post comes from the agency’s “abortion surveillance” reports, which have been published annually since 1974 (and which have included data from 1969). Its figures from 1973 through 1996 include data from all 50 states, the District of Columbia and New York City – 52 “reporting areas” in all. Since 1997, the CDC’s totals have lacked data from some states (most notably California) for the years that those states did not report data to the agency. The four reporting areas that did not submit data to the CDC in 2021 – California, Maryland, New Hampshire and New Jersey – accounted for approximately 25% of all legal induced abortions in the U.S. in 2020, according to Guttmacher’s data. Most states, though,  do  have data in the reports, and the figures for the vast majority of them came from each state’s central health agency, while for some states, the figures came from hospitals and other medical facilities.

Discussion of CDC abortion data involving women’s state of residence, marital status, race, ethnicity, age, abortion history and the number of previous live births excludes the low share of abortions where that information was not supplied. Read the methodology for the CDC’s latest abortion surveillance report , which includes data from 2021, for more details. Previous reports can be found at  stacks.cdc.gov  by entering “abortion surveillance” into the search box.

For the numbers of deaths caused by induced abortions in 1963 and 1965, this analysis looks at reports by the then-U.S. Department of Health, Education and Welfare, a precursor to the Department of Health and Human Services. In computing those figures, we excluded abortions listed in the report under the categories “spontaneous or unspecified” or as “other.” (“Spontaneous abortion” is another way of referring to miscarriages.)

Guttmacher data in this post comes from national surveys of abortion providers that Guttmacher has conducted 19 times since 1973. Guttmacher compiles its figures after contacting every known provider of abortions – clinics, hospitals and physicians’ offices – in the country. It uses questionnaires and health department data, and it provides estimates for abortion providers that don’t respond to its inquiries. (In 2020, the last year for which it has released data on the number of abortions in the U.S., it used estimates for 12% of abortions.) For most of the 2000s, Guttmacher has conducted these national surveys every three years, each time getting abortion data for the prior two years. For each interim year, Guttmacher has calculated estimates based on trends from its own figures and from other data.

The latest full summary of Guttmacher data came in the institute’s report titled “Abortion Incidence and Service Availability in the United States, 2020.” It includes figures for 2020 and 2019 and estimates for 2018. The report includes a methods section.

In addition, this post uses data from StatPearls, an online health care resource, on complications from abortion.

An exact answer is hard to come by. The CDC and the Guttmacher Institute have each tried to measure this for around half a century, but they use different methods and publish different figures.

The last year for which the CDC reported a yearly national total for abortions is 2021. It found there were 625,978 abortions in the District of Columbia and the 46 states with available data that year, up from 597,355 in those states and D.C. in 2020. The corresponding figure for 2019 was 607,720.

The last year for which Guttmacher reported a yearly national total was 2020. It said there were 930,160 abortions that year in all 50 states and the District of Columbia, compared with 916,460 in 2019.

• How the CDC gets its data: It compiles figures that are voluntarily reported by states’ central health agencies, including separate figures for New York City and the District of Columbia. Its latest totals do not include figures from California, Maryland, New Hampshire or New Jersey, which did not report data to the CDC. ( Read the methodology from the latest CDC report .)
• How Guttmacher gets its data: It compiles its figures after contacting every known abortion provider – clinics, hospitals and physicians’ offices – in the country. It uses questionnaires and health department data, then provides estimates for abortion providers that don’t respond. Guttmacher’s figures are higher than the CDC’s in part because they include data (and in some instances, estimates) from all 50 states. ( Read the institute’s latest full report and methodology .)

While the Guttmacher Institute supports abortion rights, its empirical data on abortions in the U.S. has been widely cited by  groups  and  publications  across the political spectrum, including by a  number of those  that  disagree with its positions .

These estimates from Guttmacher and the CDC are results of multiyear efforts to collect data on abortion across the U.S. Last year, Guttmacher also began publishing less precise estimates every few months , based on a much smaller sample of providers.

The figures reported by these organizations include only legal induced abortions conducted by clinics, hospitals or physicians’ offices, or those that make use of abortion pills dispensed from certified facilities such as clinics or physicians’ offices. They do not account for the use of abortion pills that were obtained  outside of clinical settings .

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The annual number of U.S. abortions rose for years after Roe v. Wade legalized the procedure in 1973, reaching its highest levels around the late 1980s and early 1990s, according to both the CDC and Guttmacher. Since then, abortions have generally decreased at what a CDC analysis called  “a slow yet steady pace.”

Guttmacher says the number of abortions occurring in the U.S. in 2020 was 40% lower than it was in 1991. According to the CDC, the number was 36% lower in 2021 than in 1991, looking just at the District of Columbia and the 46 states that reported both of those years.

(The corresponding line graph shows the long-term trend in the number of legal abortions reported by both organizations. To allow for consistent comparisons over time, the CDC figures in the chart have been adjusted to ensure that the same states are counted from one year to the next. Using that approach, the CDC figure for 2021 is 622,108 legal abortions.)

There have been occasional breaks in this long-term pattern of decline – during the middle of the first decade of the 2000s, and then again in the late 2010s. The CDC reported modest 1% and 2% increases in abortions in 2018 and 2019, and then, after a 2% decrease in 2020, a 5% increase in 2021. Guttmacher reported an 8% increase over the three-year period from 2017 to 2020.

As noted above, these figures do not include abortions that use pills obtained outside of clinical settings.

Guttmacher says that in 2020 there were 14.4 abortions in the U.S. per 1,000 women ages 15 to 44. Its data shows that the rate of abortions among women has generally been declining in the U.S. since 1981, when it reported there were 29.3 abortions per 1,000 women in that age range.

The CDC says that in 2021, there were 11.6 abortions in the U.S. per 1,000 women ages 15 to 44. (That figure excludes data from California, the District of Columbia, Maryland, New Hampshire and New Jersey.) Like Guttmacher’s data, the CDC’s figures also suggest a general decline in the abortion rate over time. In 1980, when the CDC reported on all 50 states and D.C., it said there were 25 abortions per 1,000 women ages 15 to 44.

That said, both Guttmacher and the CDC say there were slight increases in the rate of abortions during the late 2010s and early 2020s. Guttmacher says the abortion rate per 1,000 women ages 15 to 44 rose from 13.5 in 2017 to 14.4 in 2020. The CDC says it rose from 11.2 per 1,000 in 2017 to 11.4 in 2019, before falling back to 11.1 in 2020 and then rising again to 11.6 in 2021. (The CDC’s figures for those years exclude data from California, D.C., Maryland, New Hampshire and New Jersey.)

The CDC broadly divides abortions into two categories: surgical abortions and medication abortions, which involve pills. Since the Food and Drug Administration first approved abortion pills in 2000, their use has increased over time as a share of abortions nationally, according to both the CDC and Guttmacher.

The majority of abortions in the U.S. now involve pills, according to both the CDC and Guttmacher. The CDC says 56% of U.S. abortions in 2021 involved pills, up from 53% in 2020 and 44% in 2019. Its figures for 2021 include the District of Columbia and 44 states that provided this data; its figures for 2020 include D.C. and 44 states (though not all of the same states as in 2021), and its figures for 2019 include D.C. and 45 states.

Guttmacher, which measures this every three years, says 53% of U.S. abortions involved pills in 2020, up from 39% in 2017.

Two pills commonly used together for medication abortions are mifepristone, which, taken first, blocks hormones that support a pregnancy, and misoprostol, which then causes the uterus to empty. According to the FDA, medication abortions are safe  until 10 weeks into pregnancy.

Surgical abortions conducted  during the first trimester  of pregnancy typically use a suction process, while the relatively few surgical abortions that occur  during the second trimester  of a pregnancy typically use a process called dilation and evacuation, according to the UCLA School of Medicine.

In 2020, there were 1,603 facilities in the U.S. that provided abortions,  according to Guttmacher . This included 807 clinics, 530 hospitals and 266 physicians’ offices.

While clinics make up half of the facilities that provide abortions, they are the sites where the vast majority (96%) of abortions are administered, either through procedures or the distribution of pills, according to Guttmacher’s 2020 data. (This includes 54% of abortions that are administered at specialized abortion clinics and 43% at nonspecialized clinics.) Hospitals made up 33% of the facilities that provided abortions in 2020 but accounted for only 3% of abortions that year, while just 1% of abortions were conducted by physicians’ offices.

Looking just at clinics – that is, the total number of specialized abortion clinics and nonspecialized clinics in the U.S. – Guttmacher found the total virtually unchanged between 2017 (808 clinics) and 2020 (807 clinics). However, there were regional differences. In the Midwest, the number of clinics that provide abortions increased by 11% during those years, and in the West by 6%. The number of clinics  decreased  during those years by 9% in the Northeast and 3% in the South.

The total number of abortion providers has declined dramatically since the 1980s. In 1982, according to Guttmacher, there were 2,908 facilities providing abortions in the U.S., including 789 clinics, 1,405 hospitals and 714 physicians’ offices.

The CDC does not track the number of abortion providers.

In the District of Columbia and the 46 states that provided abortion and residency information to the CDC in 2021, 10.9% of all abortions were performed on women known to live outside the state where the abortion occurred – slightly higher than the percentage in 2020 (9.7%). That year, D.C. and 46 states (though not the same ones as in 2021) reported abortion and residency data. (The total number of abortions used in these calculations included figures for women with both known and unknown residential status.)

The share of reported abortions performed on women outside their state of residence was much higher before the 1973 Roe decision that stopped states from banning abortion. In 1972, 41% of all abortions in D.C. and the 20 states that provided this information to the CDC that year were performed on women outside their state of residence. In 1973, the corresponding figure was 21% in the District of Columbia and the 41 states that provided this information, and in 1974 it was 11% in D.C. and the 43 states that provided data.

In the District of Columbia and the 46 states that reported age data to  the CDC in 2021, the majority of women who had abortions (57%) were in their 20s, while about three-in-ten (31%) were in their 30s. Teens ages 13 to 19 accounted for 8% of those who had abortions, while women ages 40 to 44 accounted for about 4%.

The vast majority of women who had abortions in 2021 were unmarried (87%), while married women accounted for 13%, according to  the CDC , which had data on this from 37 states.

In the District of Columbia, New York City (but not the rest of New York) and the 31 states that reported racial and ethnic data on abortion to  the CDC , 42% of all women who had abortions in 2021 were non-Hispanic Black, while 30% were non-Hispanic White, 22% were Hispanic and 6% were of other races.

Looking at abortion rates among those ages 15 to 44, there were 28.6 abortions per 1,000 non-Hispanic Black women in 2021; 12.3 abortions per 1,000 Hispanic women; 6.4 abortions per 1,000 non-Hispanic White women; and 9.2 abortions per 1,000 women of other races, the  CDC reported  from those same 31 states, D.C. and New York City.

For 57% of U.S. women who had induced abortions in 2021, it was the first time they had ever had one,  according to the CDC.  For nearly a quarter (24%), it was their second abortion. For 11% of women who had an abortion that year, it was their third, and for 8% it was their fourth or more. These CDC figures include data from 41 states and New York City, but not the rest of New York.

Nearly four-in-ten women who had abortions in 2021 (39%) had no previous live births at the time they had an abortion,  according to the CDC . Almost a quarter (24%) of women who had abortions in 2021 had one previous live birth, 20% had two previous live births, 10% had three, and 7% had four or more previous live births. These CDC figures include data from 41 states and New York City, but not the rest of New York.

The vast majority of abortions occur during the first trimester of a pregnancy. In 2021, 93% of abortions occurred during the first trimester – that is, at or before 13 weeks of gestation,  according to the CDC . An additional 6% occurred between 14 and 20 weeks of pregnancy, and about 1% were performed at 21 weeks or more of gestation. These CDC figures include data from 40 states and New York City, but not the rest of New York.

About 2% of all abortions in the U.S. involve some type of complication for the woman , according to an article in StatPearls, an online health care resource. “Most complications are considered minor such as pain, bleeding, infection and post-anesthesia complications,” according to the article.

The CDC calculates  case-fatality rates for women from induced abortions – that is, how many women die from abortion-related complications, for every 100,000 legal abortions that occur in the U.S .  The rate was lowest during the most recent period examined by the agency (2013 to 2020), when there were 0.45 deaths to women per 100,000 legal induced abortions. The case-fatality rate reported by the CDC was highest during the first period examined by the agency (1973 to 1977), when it was 2.09 deaths to women per 100,000 legal induced abortions. During the five-year periods in between, the figure ranged from 0.52 (from 1993 to 1997) to 0.78 (from 1978 to 1982).

The CDC calculates death rates by five-year and seven-year periods because of year-to-year fluctuation in the numbers and due to the relatively low number of women who die from legal induced abortions.

In 2020, the last year for which the CDC has information , six women in the U.S. died due to complications from induced abortions. Four women died in this way in 2019, two in 2018, and three in 2017. (These deaths all followed legal abortions.) Since 1990, the annual number of deaths among women due to legal induced abortion has ranged from two to 12.

The annual number of reported deaths from induced abortions (legal and illegal) tended to be higher in the 1980s, when it ranged from nine to 16, and from 1972 to 1979, when it ranged from 13 to 63. One driver of the decline was the drop in deaths from illegal abortions. There were 39 deaths from illegal abortions in 1972, the last full year before Roe v. Wade. The total fell to 19 in 1973 and to single digits or zero every year after that. (The number of deaths from legal abortions has also declined since then, though with some slight variation over time.)

The number of deaths from induced abortions was considerably higher in the 1960s than afterward. For instance, there were 119 deaths from induced abortions in  1963  and 99 in  1965 , according to reports by the then-U.S. Department of Health, Education and Welfare, a precursor to the Department of Health and Human Services. The CDC is a division of Health and Human Services.

Note: This is an update of a post originally published May 27, 2022, and first updated June 24, 2022.

Support for legal abortion is widespread in many countries, especially in Europe

Nearly a year after roe’s demise, americans’ views of abortion access increasingly vary by where they live, by more than two-to-one, americans say medication abortion should be legal in their state, most latinos say democrats care about them and work hard for their vote, far fewer say so of gop, positive views of supreme court decline sharply following abortion ruling, most popular.

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What is ADHD?

Signs and symptoms.

• Managing Symptoms

ADHD in Adults

More information.

ADHD is one of the most common neurodevelopmental disorders of childhood. It is usually first diagnosed in childhood and often lasts into adulthood. Children with ADHD may have trouble paying attention, controlling impulsive behaviors (may act without thinking about what the result will be), or be overly active.

It is normal for children to have trouble focusing and behaving at one time or another. However, children with ADHD do not just grow out of these behaviors. The symptoms continue, can be severe, and can cause difficulty at school, at home, or with friends.

A child with ADHD might:

• daydream a lot
• forget or lose things a lot
• squirm or fidget
• talk too much
• make careless mistakes or take unnecessary risks
• have a hard time resisting temptation
• have trouble taking turns
• have difficulty getting along with others

Learn more about signs and symptoms

Get information and support from the National Resource Center on ADHD

There are three different ways ADHD presents itself, depending on which types of symptoms are strongest in the individual:

• Predominantly Inattentive Presentation: It is hard for the individual to organize or finish a task, to pay attention to details, or to follow instructions or conversations. The person is easily distracted or forgets details of daily routines.
• Predominantly Hyperactive-Impulsive Presentation: The person fidgets and talks a lot. It is hard to sit still for long (e.g., for a meal or while doing homework). Smaller children may run, jump or climb constantly. The individual feels restless and has trouble with impulsivity. Someone who is impulsive may interrupt others a lot, grab things from people, or speak at inappropriate times. It is hard for the person to wait their turn or listen to directions. A person with impulsiveness may have more accidents and injuries than others.
• Combined Presentation: Symptoms of the above two types are equally present in the person.

Because symptoms can change over time, the presentation may change over time as well.

 Learn about symptoms of ADHD, how ADHD is diagnosed, and treatment recommendations including behavior therapy, medication, and school support.

Causes of ADHD

Scientists are studying cause(s) and risk factors in an effort to find better ways to manage and reduce the chances of a person having ADHD. The cause(s) and risk factors for ADHD are unknown, but current research shows that genetics plays an important role. Recent studies link genetic factors with ADHD. 1

In addition to genetics, scientists are studying other possible causes and risk factors including:

• Brain injury
• Exposure to environmental risks (e.g., lead) during pregnancy or at a young age
• Alcohol and tobacco use during pregnancy
• Premature delivery
• Low birth weight

Research does not support the popularly held views that ADHD is caused by eating too much sugar, watching too much television, parenting, or social and environmental factors such as poverty or family chaos. Of course, many things, including these, might make symptoms worse, especially in certain people. But the evidence is not strong enough to conclude that they are the main causes of ADHD.

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Deciding if a child has ADHD is a process with several steps. There is no single test to diagnose ADHD, and many other problems, like anxiety, depression, sleep problems, and certain types of learning disabilities, can have similar symptoms. One step of the process involves having a medical exam, including hearing and vision tests , to rule out other problems with symptoms like ADHD. Diagnosing ADHD usually includes a checklist for rating ADHD symptoms and taking a history of the child from parents, teachers, and sometimes, the child.

Learn more about the criteria for diagnosing ADHD

In most cases, ADHD is best treated with a combination of behavior therapy and medication. For preschool-aged children (4-5 years of age) with ADHD, behavior therapy, particularly training for parents, is recommended as the first line of treatment before medication is tried. What works best can depend on the child and family. Good treatment plans will include close monitoring, follow-ups, and making changes, if needed, along the way.

Learn more about treatments

Managing Symptoms: Staying Healthy

Being healthy is important for all children and can be especially important for children with ADHD. In addition to behavioral therapy and medication, having a healthy lifestyle can make it easier for your child to deal with ADHD symptoms. Here are some healthy behaviors that may help:

• Developing healthy eating habits  such as eating plenty of fruits, vegetables, and whole grains and choosing lean protein sources
• Participating in daily  physical activity based on age
• Limiting the amount of daily screen time from TVs, computers, phones, and other electronics
• Getting the recommended amount of sleep each night based on age

If you or your doctor has concerns about ADHD, you can take your child to a specialist such as a child psychologist, child psychiatrist, or developmental pediatrician, or you can contact your local early intervention agency (for children under 3) or public school (for children 3 and older).

The Centers for Disease Control and Prevention (CDC) funds the National Resource Center on ADHD , a program of CHADD – Children and Adults with Attention-Deficit/Hyperactivity Disorder. Their website has links to information for people with ADHD and their families. The National Resource Center operates a call center (1-866-200-8098) with trained staff to answer questions about ADHD.

For more information on services for children with special needs, visit the Center for Parent Information and Resources.  To find the Parent Center near you, you can visit this website.

ADHD can last into adulthood. Some adults have ADHD but have never been diagnosed. The symptoms can cause difficulty at work, at home, or with relationships. Symptoms may look different at older ages, for example, hyperactivity may appear as extreme restlessness. Symptoms can become more severe when the demands of adulthood increase. For more information about diagnosis and treatment throughout the lifespan, please visit the websites of the National Resource Center on ADHD  and the National Institutes of Mental Health .

• National Resource Center on ADHD
• National Institute of Mental Health (NIMH)
• Faraone, S. V., Banaschewski, T., Coghill, D., Zheng, Y., Biederman, J., Bellgrove, M. A., . . . Wang, Y. (2021). The World Federation of ADHD International Consensus Statement: 208 evidence-based conclusions about the disorder. Neuroscience & Biobehavioral Reviews. doi:10.1016/j.neubiorev.2021.01.022

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