types of research studies pyramid

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Evidence-Based Research: Levels of Evidence Pyramid

Introduction.

One way to organize the different types of evidence involved in evidence-based practice research is the levels of evidence pyramid. The pyramid includes a variety of evidence types and levels.

• systematic reviews
• critically-appraised topics
• critically-appraised individual articles
• randomized controlled trials
• cohort studies
• case-controlled studies, case series, and case reports
• Background information, expert opinion

Levels of evidence pyramid

The levels of evidence pyramid provides a way to visualize both the quality of evidence and the amount of evidence available. For example, systematic reviews are at the top of the pyramid, meaning they are both the highest level of evidence and the least common. As you go down the pyramid, the amount of evidence will increase as the quality of the evidence decreases.

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EBM Pyramid and EBM Page Generator, copyright 2006 Trustees of Dartmouth College and Yale University. All Rights Reserved. Produced by Jan Glover, David Izzo, Karen Odato and Lei Wang.

Filtered Resources

Filtered resources appraise the quality of studies and often make recommendations for practice. The main types of filtered resources in evidence-based practice are:

Scroll down the page to the Systematic reviews , Critically-appraised topics , and Critically-appraised individual articles sections for links to resources where you can find each of these types of filtered information.

Systematic reviews

Authors of a systematic review ask a specific clinical question, perform a comprehensive literature review, eliminate the poorly done studies, and attempt to make practice recommendations based on the well-done studies. Systematic reviews include only experimental, or quantitative, studies, and often include only randomized controlled trials.

You can find systematic reviews in these filtered databases :

• Cochrane Database of Systematic Reviews Cochrane systematic reviews are considered the gold standard for systematic reviews. This database contains both systematic reviews and review protocols. To find only systematic reviews, select Cochrane Reviews in the Document Type box.
• JBI EBP Database (formerly Joanna Briggs Institute EBP Database) This database includes systematic reviews, evidence summaries, and best practice information sheets. To find only systematic reviews, click on Limits and then select Systematic Reviews in the Publication Types box. To see how to use the limit and find full text, please see our Joanna Briggs Institute Search Help page .

You can also find systematic reviews in this unfiltered database :

To learn more about finding systematic reviews, please see our guide:

• Filtered Resources: Systematic Reviews

Critically-appraised topics

Authors of critically-appraised topics evaluate and synthesize multiple research studies. Critically-appraised topics are like short systematic reviews focused on a particular topic.

You can find critically-appraised topics in these resources:

• Annual Reviews This collection offers comprehensive, timely collections of critical reviews written by leading scientists. To find reviews on your topic, use the search box in the upper-right corner.
• Guideline Central This free database offers quick-reference guideline summaries organized by a new non-profit initiative which will aim to fill the gap left by the sudden closure of AHRQ’s National Guideline Clearinghouse (NGC).
• JBI EBP Database (formerly Joanna Briggs Institute EBP Database) To find critically-appraised topics in JBI, click on Limits and then select Evidence Summaries from the Publication Types box. To see how to use the limit and find full text, please see our Joanna Briggs Institute Search Help page .
• National Institute for Health and Care Excellence (NICE) Evidence-based recommendations for health and care in England.
• Filtered Resources: Critically-Appraised Topics

Critically-appraised individual articles

Authors of critically-appraised individual articles evaluate and synopsize individual research studies.

You can find critically-appraised individual articles in these resources:

• EvidenceAlerts Quality articles from over 120 clinical journals are selected by research staff and then rated for clinical relevance and interest by an international group of physicians. Note: You must create a free account to search EvidenceAlerts.
• ACP Journal Club This journal publishes reviews of research on the care of adults and adolescents. You can either browse this journal or use the Search within this publication feature.
• Evidence-Based Nursing This journal reviews research studies that are relevant to best nursing practice. You can either browse individual issues or use the search box in the upper-right corner.

To learn more about finding critically-appraised individual articles, please see our guide:

• Filtered Resources: Critically-Appraised Individual Articles

Unfiltered resources

You may not always be able to find information on your topic in the filtered literature. When this happens, you'll need to search the primary or unfiltered literature. Keep in mind that with unfiltered resources, you take on the role of reviewing what you find to make sure it is valid and reliable.

Note: You can also find systematic reviews and other filtered resources in these unfiltered databases.

The Levels of Evidence Pyramid includes unfiltered study types in this order of evidence from higher to lower:

You can search for each of these types of evidence in the following databases:

TRIP database

Background information & expert opinion.

Background information and expert opinions are not necessarily backed by research studies. They include point-of-care resources, textbooks, conference proceedings, etc.

• Family Physicians Inquiries Network: Clinical Inquiries Provide the ideal answers to clinical questions using a structured search, critical appraisal, authoritative recommendations, clinical perspective, and rigorous peer review. Clinical Inquiries deliver best evidence for point-of-care use.
• Harrison, T. R., & Fauci, A. S. (2009). Harrison's Manual of Medicine . New York: McGraw-Hill Professional. Contains the clinical portions of Harrison's Principles of Internal Medicine .
• Lippincott manual of nursing practice (8th ed.). (2006). Philadelphia, PA: Lippincott Williams & Wilkins. Provides background information on clinical nursing practice.
• Medscape: Drugs & Diseases An open-access, point-of-care medical reference that includes clinical information from top physicians and pharmacists in the United States and worldwide.
• Virginia Henderson Global Nursing e-Repository An open-access repository that contains works by nurses and is sponsored by Sigma Theta Tau International, the Honor Society of Nursing. Note: This resource contains both expert opinion and evidence-based practice articles.
• Previous Page: Phrasing Research Questions
• Next Page: Evidence Types
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• Volume 21, Issue 4
• New evidence pyramid
• Article Text
• Article info
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• M Hassan Murad ,
• Mouaz Alsawas ,
• http://orcid.org/0000-0001-5481-696X Fares Alahdab
• Rochester, Minnesota , USA
• Correspondence to : Dr M Hassan Murad, Evidence-based Practice Center, Mayo Clinic, Rochester, MN 55905, USA; murad.mohammad{at}mayo.edu

https://doi.org/10.1136/ebmed-2016-110401

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• EDUCATION & TRAINING (see Medical Education & Training)
• EPIDEMIOLOGY
• GENERAL MEDICINE (see Internal Medicine)

The first and earliest principle of evidence-based medicine indicated that a hierarchy of evidence exists. Not all evidence is the same. This principle became well known in the early 1990s as practising physicians learnt basic clinical epidemiology skills and started to appraise and apply evidence to their practice. Since evidence was described as a hierarchy, a compelling rationale for a pyramid was made. Evidence-based healthcare practitioners became familiar with this pyramid when reading the literature, applying evidence or teaching students.

Various versions of the evidence pyramid have been described, but all of them focused on showing weaker study designs in the bottom (basic science and case series), followed by case–control and cohort studies in the middle, then randomised controlled trials (RCTs), and at the very top, systematic reviews and meta-analysis. This description is intuitive and likely correct in many instances. The placement of systematic reviews at the top had undergone several alterations in interpretations, but was still thought of as an item in a hierarchy. 1 Most versions of the pyramid clearly represented a hierarchy of internal validity (risk of bias). Some versions incorporated external validity (applicability) in the pyramid by either placing N-1 trials above RCTs (because their results are most applicable to individual patients 2 ) or by separating internal and external validity. 3

Another version (the 6S pyramid) was also developed to describe the sources of evidence that can be used by evidence-based medicine (EBM) practitioners for answering foreground questions, showing a hierarchy ranging from studies, synopses, synthesis, synopses of synthesis, summaries and systems. 4 This hierarchy may imply some sort of increasing validity and applicability although its main purpose is to emphasise that the lower sources of evidence in the hierarchy are least preferred in practice because they require more expertise and time to identify, appraise and apply.

The traditional pyramid was deemed too simplistic at times, thus the importance of leaving room for argument and counterargument for the methodological merit of different designs has been emphasised. 5 Other barriers challenged the placement of systematic reviews and meta-analyses at the top of the pyramid. For instance, heterogeneity (clinical, methodological or statistical) is an inherent limitation of meta-analyses that can be minimised or explained but never eliminated. 6 The methodological intricacies and dilemmas of systematic reviews could potentially result in uncertainty and error. 7 One evaluation of 163 meta-analyses demonstrated that the estimation of treatment outcomes differed substantially depending on the analytical strategy being used. 7 Therefore, we suggest, in this perspective, two visual modifications to the pyramid to illustrate two contemporary methodological principles ( figure 1 ). We provide the rationale and an example for each modification.

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The proposed new evidence-based medicine pyramid. (A) The traditional pyramid. (B) Revising the pyramid: (1) lines separating the study designs become wavy (Grading of Recommendations Assessment, Development and Evaluation), (2) systematic reviews are ‘chopped off’ the pyramid. (C) The revised pyramid: systematic reviews are a lens through which evidence is viewed (applied).

Rationale for modification 1

In the early 2000s, the Grading of Recommendations Assessment, Development and Evaluation (GRADE) Working Group developed a framework in which the certainty in evidence was based on numerous factors and not solely on study design which challenges the pyramid concept. 8 Study design alone appears to be insufficient on its own as a surrogate for risk of bias. Certain methodological limitations of a study, imprecision, inconsistency and indirectness, were factors independent from study design and can affect the quality of evidence derived from any study design. For example, a meta-analysis of RCTs evaluating intensive glycaemic control in non-critically ill hospitalised patients showed a non-significant reduction in mortality (relative risk of 0.95 (95% CI 0.72 to 1.25) 9 ). Allocation concealment and blinding were not adequate in most trials. The quality of this evidence is rated down due to the methodological imitations of the trials and imprecision (wide CI that includes substantial benefit and harm). Hence, despite the fact of having five RCTs, such evidence should not be rated high in any pyramid. The quality of evidence can also be rated up. For example, we are quite certain about the benefits of hip replacement in a patient with disabling hip osteoarthritis. Although not tested in RCTs, the quality of this evidence is rated up despite the study design (non-randomised observational studies). 10

Rationale for modification 2

Another challenge to the notion of having systematic reviews on the top of the evidence pyramid relates to the framework presented in the Journal of the American Medical Association User's Guide on systematic reviews and meta-analysis. The Guide presented a two-step approach in which the credibility of the process of a systematic review is evaluated first (comprehensive literature search, rigorous study selection process, etc). If the systematic review was deemed sufficiently credible, then a second step takes place in which we evaluate the certainty in evidence based on the GRADE approach. 11 In other words, a meta-analysis of well-conducted RCTs at low risk of bias cannot be equated with a meta-analysis of observational studies at higher risk of bias. For example, a meta-analysis of 112 surgical case series showed that in patients with thoracic aortic transection, the mortality rate was significantly lower in patients who underwent endovascular repair, followed by open repair and non-operative management (9%, 19% and 46%, respectively, p<0.01). Clearly, this meta-analysis should not be on top of the pyramid similar to a meta-analysis of RCTs. After all, the evidence remains consistent of non-randomised studies and likely subject to numerous confounders.

Therefore, the second modification to the pyramid is to remove systematic reviews from the top of the pyramid and use them as a lens through which other types of studies should be seen (ie, appraised and applied). The systematic review (the process of selecting the studies) and meta-analysis (the statistical aggregation that produces a single effect size) are tools to consume and apply the evidence by stakeholders.

Implications and limitations

Changing how systematic reviews and meta-analyses are perceived by stakeholders (patients, clinicians and stakeholders) has important implications. For example, the American Heart Association considers evidence derived from meta-analyses to have a level ‘A’ (ie, warrants the most confidence). Re-evaluation of evidence using GRADE shows that level ‘A’ evidence could have been high, moderate, low or of very low quality. 12 The quality of evidence drives the strength of recommendation, which is one of the last translational steps of research, most proximal to patient care.

One of the limitations of all ‘pyramids’ and depictions of evidence hierarchy relates to the underpinning of such schemas. The construct of internal validity may have varying definitions, or be understood differently among evidence consumers. A limitation of considering systematic review and meta-analyses as tools to consume evidence may undermine their role in new discovery (eg, identifying a new side effect that was not demonstrated in individual studies 13 ).

This pyramid can be also used as a teaching tool. EBM teachers can compare it to the existing pyramids to explain how certainty in the evidence (also called quality of evidence) is evaluated. It can be used to teach how evidence-based practitioners can appraise and apply systematic reviews in practice, and to demonstrate the evolution in EBM thinking and the modern understanding of certainty in evidence.

• Leibovici L
• Agoritsas T ,
• Vandvik P ,
• Neumann I , et al
• ↵ Resources for Evidence-Based Practice: The 6S Pyramid. Secondary Resources for Evidence-Based Practice: The 6S Pyramid Feb 18, 2016 4:58 PM. http://hsl.mcmaster.libguides.com/ebm
• Vandenbroucke JP
• Berlin JA ,
• Dechartres A ,
• Altman DG ,
• Trinquart L , et al
• Guyatt GH ,
• Vist GE , et al
• Coburn JA ,
• Coto-Yglesias F , et al
• Sultan S , et al
• Montori VM ,
• Ioannidis JP , et al
• Altayar O ,
• Bennett M , et al
• Nissen SE ,

Contributors MHM conceived the idea and drafted the manuscript. FA helped draft the manuscript and designed the new pyramid. MA and NA helped draft the manuscript.

Competing interests None declared.

Provenance and peer review Not commissioned; externally peer reviewed.

Linked Articles

• Editorial Pyramids are guides not rules: the evolution of the evidence pyramid Terrence Shaneyfelt BMJ Evidence-Based Medicine 2016; 21 121-122 Published Online First: 12 Jul 2016. doi: 10.1136/ebmed-2016-110498
• Perspective EBHC pyramid 5.0 for accessing preappraised evidence and guidance Brian S Alper R Brian Haynes BMJ Evidence-Based Medicine 2016; 21 123-125 Published Online First: 20 Jun 2016. doi: 10.1136/ebmed-2016-110447

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Research and Evidence-based Practice: Levels of Evidence and Study Designs

• MCHS Published Research
• Levels of Evidence and Study Designs
• Searching for the Evidence & Critical Appraisal
• Reference and Citation Management
• Writing and Publication
• Searching Grey Literature

Evidence Pyramid

An  evidence pyramid   visually depicts the evidential strength of different research designs. The image below is one of several available renderings of an evidence pyramid. Studies with the highest internal validity, characterized by a high degree of quantitative analysis, review, analysis, and stringent scientific methodoloy, are at the top of the pyramid. Observational research and expert opinion reside at the bottom of the pyramid. 

Which Research Designs for Which Questions?

Different types of research studies are better suited to answer different categories of clinical questions. You might not always find the highest level of evidence (i.e., systematic review or meta-analysis) to answer your question.  When this happens, work your way down the Evidence Pyramid to the next highest level of evidence.

Therapy : Which treatment does more harm than good?

RCT > Cohort Study  >  Case Control > Case Series

Diagnosis : Which diagnostic test should I use?

Prospective, blind comparison to a gold standard, ie. A controlled trial that looks at patients with varying degrees of an illness and administers both diagnostic tests -- the test under investigation and the "gold standard" test -- to all of the patients in the study group.

Prognosis : What is the patient's likely clinical course over time?

Cohort Study > Case Control > Case Series

Etiology / Harm : What are the causes of this disease or condition?

RCT > Cohort Study > Case Control > Case Series

Prevention : How do we reduce the chance of disease by identifying and modifying risk factors?

RCT > Cohort Study > Case Control > Case Series

Cost : Is one intervention more cost-effective than another?

Economic Analysis

Quality of Life : What will be the patient's quality of life following an intervention?

Qualitative Study

Levels of Evidence

Types of study designs.

Systematic Review:    A summary of the clinical literature. A systematic review is a critical assessment and evaluation of all research studies that address a particular clinical issue. The researchers use an organized method of locating, assembling, and evaluating a body of literature on a particular topic using a set of specific criteria. A systematic review typically includes a description of the findings of the collection of research studies. Cochrane Reviews are the gold standard!  (AHRQ Glossary of Terms)

Meta-Analysis :   A work consisting of studies using a quantitative method of combining the results of independent studies (usually drawn from the published literature) and synthesizing summaries and conclusions which may be used to evaluate therapeutic effectiveness, plan new studies, etc. It is often an overview of clinical trials. It is usually called a meta-analysis by the author or sponsoring body and should be differentiated from reviews of literature. (PubMed)

Evidence Guideline:   Systematically developed statement to assist practitioner and patient decisions about appropriate health care for specific clinical circumstances (Institute of Medicine).  These have a rigorous development process.  An example is AHRQ Guidelines at guidelines.gov or Lippincott  Procedures .

Evidence Summary:   A summary of the evidence.

Randomized Controlled Trial:    A controlled clinical trial that randomly (by chance) assigns participants to two or more groups. There are various methods to randomize study participants to their groups. (AHRQ Glossary of Terms)

Controlled Clinical Trial:   A type of clinical trial comparing the effectiveness of one medication or treatment with the effectiveness of another medication or treatment. In many controlled trials, the other treatment is a placebo (inactive substance) and is considered the "control." (AHRQ Glossary of Terms)

Cohort Study:   A clinical research study in which people who presently have a certain condition or receive a particular treatment are followed over time and compared with another group of people who are not affected by the condition. (AHRQ Glossary of Terms)

Case Control Study :    The observational epidemiologic study of persons with the disease (or other outcome variable) of interest and a suitable control (comparison, reference) group of persons without the disease. The relationship of an attribute to the disease is examined by comparing the diseased and nondiseased with regard to how frequently the attribute is present or, if quantitative, the levels of the attribute, in each of the groups. (OCEBM Table of Evidence Glossary)

Case Series:   A group or series of case reports involving patients who were given similar treatment. Reports of case series usually contain detailed information about the individual patients. This includes demographic information (for example, age, gender, ethnic origin) and information on diagnosis, treatment, response to treatment, and follow-up after treatment. (OCEBM Table of Evidence Glossary)

Case Study :    An investigation of a single subject or a single unit, which could be a small number of individuals who seem to be representative of a larger group or very different from it. (Dictionary of Nursing Theory and Research, Fourth Edition)

Editorial:    Work consisting of a statement of the opinions, beliefs, and policy of the editor or publisher of a journal, usually on current matters of medical or scientific significance to the medical community or society at large. The editorials published by editors of journals representing the official organ of a society or organization are generally substantive. (PubMed)

Opinion:   A belief or conclusion held with confidence but not substantiated by positive knowledge or proof. (The Free Dictionary)

Animal Research:   A laboratory experiment using animals to study the development and progression of diseases. Animal studies also test how safe and effective new treatments are before they are tested in people.(NCI Dictionary of Cancer Terms)

In Vitro Research:   In the laboratory (outside the body). The opposite of in vivo (in the body). (NCI Dictionary of Cancer Terms)

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Evidence Hierarchy: What is the Best Evidence?

Now that we have learned about the different types of resources – filtered, unfiltered, and background – let’s take a look at the evidence hierarchy (also known as the levels of evidence).

The evidence hierarchy pyramid is a visual representation of the strength of different research study designs. It can be helpful to think about evidence as a pyramid – not all study designs and resource types are created equal.

Filtered Information

At the top of the pyramid, we have filtered information – this includes systematic reviews, meta-analyses, and evidence syntheses; practice guidelines; and critically-appraised topics found in clinical resources. This type of information has used a high-quality methodology to synthesize primary resources – meaning that they have searched for available primary literature and evaluated its validity to provide answers to specific clinical questions. It is important to remember that the quality and reliability of filtered information can only be as good as the primary literature it includes.

Unfiltered Information

In the middle of the pyramid, we have unfiltered information – this is known as primary literature. These are individual experimental study designs. A randomized controlled trial is considered the highest quality individual study design, followed by cohort studies and case-controlled studies. We will discuss these study designs in more detail later in the tutorial.

Background Information

At the base of the pyramid, we have background information and expert opinion . Background information is not typically used in making complex clinical decisions, but can be helpful in defining parts of your clinical question.

Evidence-Based Practice Copyright © by Various Authors - See Each Chapter Attribution is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License , except where otherwise noted.

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Research Help

The evidence pyramid.

Acquiring the most credible and relevant studies for a specific research question is the second step of evidence-based practice. Use the evidence pyramid to help you to determine the best study types to answer your PICO clinical question .

Levels of Evidence

Studies can be categorized and assigned a level based on the strength of the evidence and methodology. The levels of evidence can be visualized as a pyramid, with the strongest and most reliable evidence at the top and weaker evidence at the bottom. The amount of available evidence increases as you move down the pyramid, but decreases in quality.

Different types of clinical questions are best answered by different study types.  You might not always find the highest level of evidence to answer your question. When this happens, work your way down to the next highest level of evidence.

Filtered Evidence

The top of the pyramid represents the strongest evidence. These studies evaluate and synthesize the evidence for you.

• Practice guidelines Recommendations about best practice. They explain how to diagnose and manage a medical condition based on the evidence presented in higher level subject research such as systematic reviews, meta-analyses or comprehensive original research studies.
• Meta-analyses A meta-analysis is a report combining multiple studies on the same question in order to validate and strengthen conclusions using statistical analysis.
• Systematic reviews A systematic review seeks to answer a specific research question by identifying, appraising and synthesizing previously published research.

To find filtered evidence, search databases for secondary source types.

Unfiltered Evidence

Further down the pyramid are studies that you will need to read, interpret and evaluate the application to practice.

• Randomized controlled trials A randomized controlled trial (RCT) is a clinical trial where participants are randomly assigned to either a treatment group or a control group. The test-treatment group receives the treatment being studied while the control group receives either an alternative treatment, no treatment, or a placebo. Effects of the treatments are monitored to determine the efficacy of the treatment while reducing bias in both researchers and participants.
• Cohort studies A cohort study is an observational study of groups selected by their exposure to factors hypothesized to influence occurrence of a particular disease or other outcome.
• Case control studies Case control studies are comparisons that start with the identification of persons with the disease or outcome of interest and a control (comparison, referent) group without the disease or outcome of interest. The relationship of an attribute is examined by comparing both groups with regard to the frequency or levels of outcome over time.
• Case series and case reports These are detailed reports of the symptoms, signs, diagnosis, treatment, and follow-up of an individual patient that usually describes an unusual or novel occurrence. Case reports are one of the cornerstones of medical progress and provide new ideas in medicine.
• Background information & expert opinion Information that can often be found in point-of-care resources, medical textbooks, handbooks, encyclopedias, and reference materials. Background information is important to consult when you need general information about a condition, medication, or diagnostic test. Expert opinion is a belief or interpretation by specialists with experience in a specific area.

To find unfiltered evidence, search databases for primary source types.

Questions & Help

If you have questions on this, or another, topic, contact a librarian for help!

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

Evidence-Based Practice: Evidence Pyramid

• What is Evidence-Based Medicine?

Evidence Pyramid

• Rating Evidence
• EBP by Subject Area

Understanding the Evidence Pyramid

Studies are assigned levels of evidence based on their methodology. The evidence pyramid is an easy way to visualize this hierarchy of evidence.

At the top of the pyramid is filtered evidence including systematic reviews, meta-analyses, and critical appraisals.  These studies evaluate and synthesize the literature.  The top of the pyramid represents the strongest evidence.

At the base of the pyramid is unfiltered evidence including randomized controlled trials, cohort studies and case reports. These are individual reports and studies, also known as the primary literature. 

You should seek the highest level of evidence available, but remember that evidence at the top of the pyramid might not exist for your particular clinical question. If that is the case, you'll need to move down the pyramid to find the strongest evidence that addresses your clinical question.

Use the following databases to search for evidence-based research. Click on "More" for directions on how to use filters to retrieve a specific study type.

• Campbell Collaboration
• CINAHL Ultimate This link opens in a new window
• Cochrane Databases (OVID) This link opens in a new window
• Cochrane (Wiley Publishing) abstract only
• Joanna Briggs Institute This link opens in a new window
• Medline Ultimate (Ebsco) This link opens in a new window more... less... https://search.ebscohost.com/login.aspx?authtype=ip,shib&custid=stimson&groupid=main&profile=ehost&defaultdb=mdc
• Medline (Ovid) This link opens in a new window more... less... Run your search, then select "Additional Limits". Under the Publication Type heading, select the type of study you desire such as a systematic review, meta-analysis or randomized controlled trial.
• Medline (PubMed) This link opens in a new window more... less... Run your search, then select the desired type of study under the Article Types limit on the left sidebar. You first must customize the studies you wish to appear on the sidebar.
• Medline (PubMed) Clinical Queries This link opens in a new window
• Medline (PubMed) NLM PICO Search
• Nursing and Allied Health: Comprehensive This link opens in a new window
• Ovid Emcare This link opens in a new window
• PEDro This link opens in a new window
• PROSPERO This link opens in a new window
• TRIP Database This link opens in a new window

Top of the Pyramid - Filtered Evidence - Meta Analyses and Systematic Reviews

A systematic review focuses on a clinical topic and answers a specific question.  A thorough literature search is conducted to identify all studies using sound methodology.  The studies are evaluated and the results are summarized according to preselected criteria. No quantitative statistical analysis is done.

A meta analysis carefully examines a number of credible studies on a topic and combines the results using quantitative statistical methodology. It is a higher quality systematic review and therefore is at the top of the pyramid.

Use the databases above and limit your search to systematic review and meta analysis.

The best bet for systematic reviews is the Campbell Collaboration for the social sciences and the Cochrane Database for medical and social science topics.  The Cochrane through Wiley is the best interface even though it is not full text.

Top of the Pyramid - Filtered Evidence - CATs

A Critically Appraised Topic (CAT) is a short summary of the best available evidence, created to answer a specific clinical question.

A Critically Appraised Topic can be found in the following databases:

• BestBETS (Best Evidence Topics)(Manchester Royal Infirmary, UK) more... less... .
• Dynamed Plus This link opens in a new window more... less... This solution combines top evidence-based drug and disease information in a single solution to support your clinical decision-making needs at the point of care. It covers all specialties; provides clear, graded evidence summaries, calculators, images; includes CME and MOC credits, and the depth and breadth of drug information capabilities of IBM Micromedex.
• Essential Evidence Plus This link opens in a new window
• National Guideline Clearinghouse This link opens in a new window
• UpToDate This link opens in a new window more... less... To maintain uninteruppted access to UpToDate, go to https://www.uptodate.com/home/maintaining-access

Top of the Pyramid - Filtered Evidence - CAIAs

A Critically-Appraised Individual Article (CAIAs) identifies, evaluates and synposizes a study of excellent quality that will influence a standard of practice.

A Critically Appraised Individual Article can be found in ACP Journal Club which is located in the following database:

• EvidenceAlerts

Base of the Pyramid - Unfiltered Evidence

A randomized controlled trial is a carefully planned project that studies the effect of a therapy on patients. It includes methodologies that reduce the potential for bias (randomization and blinding) and that allow for a comparison between intervention groups and control groups.

A cohort study takes a large population and follows the patients who have a specific condition or receive a particular treatment over time and compares them with another group that has not been affected by the condition or treatment being studied. Cohort studies are observational and not as reliable as randomized controlled studies since the two groups may differ in ways other than in the variable under study.

A case control study is an observational study in which patients who already have a specific condition (case group) are compared with people who do not (control group). There is no randomization. These studies are usually less reliable than randomized controlled trials and cohort studies because showing a statistical relationship does not mean that one factor necessarily caused the other. These studies look back into time and patient records are usually reviewed.

A case report consists of collections of reports on the treatment of individual patients or a report on a single patient. Because they are reports of cases and use no control groups with which to compare outcomes, they have no statistical validity.

The Bio-Medical Library at the University of California Irvine   has created an excellent visual guide to basic research study design.

Evidence-Based Practice Tutorials

• "Find It Fast" tutorial series on evidence-based practice more... less... Find It Fast Video #1: Introduction The first episode of the evidence-based practice information resources video tutorial series. The basic steps for finding clinical information, especially how to construct a focused, answerable clinical question, are reviewed. Find It Fast Video #2: The Clinical Question Your choice of clinical information resource is closely related to the nature of the clinical question you ask. This tutorial covers various types of clinical questions and how they are associated with different types of research studies.
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Evidence-Based Practice

• EBP: An Introduction
• EBP Tutorials
• Terminology

Evidence Pyramid

Types of study designs.

• Which Research Designs for Which Question?
• Sources of Evidence
• CINAHL & PubMed
• Critical Appraisal
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Medical scientists and practitioners rank evidence according to its quality. When these types of evidence are ranked as levels, one on top of the other, the resulting image takes the form of a pyramid, because the higher the quality of evidence, the rarer it is, and the lower the quality of evidence, the more ubiquitous it is. The highest quality evidence (level 1 evidence) is the systematic review:

University of Louisville, 2018.

• Levels of Evidence Table - CEBM A table from Oxford Centre for Evidence-Based Medicine explaining the levels of evidence.

Systematic Review  A summary of the clinical literature. A systematic review is a critical assessment and evaluation of all research studies that address a particular clinical issue. The researchers use an organized method of locating, assembling, and evaluating a body of literature on a particular topic using a set of specific criteria. A systematic review typically includes a description of the findings of the collection of research studies. (AHRQ Glossary of Terms)

Meta-Analysis   A work consisting of studies using a quantitative method of combining the results of independent studies (usually drawn from the published literature) and synthesizing summaries and conclusions which may be used to evaluate therapeutic effectiveness, plan new studies, etc. It is often an overview of clinical trials. It is usually called a meta-analysis by the author or sponsoring body and should be differentiated from reviews of literature. (PubMed)

Randomized Controlled Trial   A controlled clinical trial that randomly (by chance) assigns participants to two or more groups. There are various methods to randomize study participants to their groups. (AHRQ Glossary of Terms)

Controlled Clinical Trial  A type of clinical trial comparing the effectiveness of one medication or treatment with the effectiveness of another medication or treatment. In many controlled trials, the other treatment is a placebo (inactive substance) and is considered the "control." (AHRQ Glossary of Terms)

Cohort Study   A clinical research study in which people who presently have a certain condition or receive a particular treatment are followed over time and compared with another group of people who are not affected by the condition. (AHRQ Glossary of Terms)

Case Control Study The observational epidemiologic study of persons with the disease (or other outcome variable) of interest and a suitable control (comparison, reference) group of persons without the disease. The relationship of an attribute to the disease is examined by comparing the diseased and nondiseased with regard to how frequently the attribute is present or, if quantitative, the levels of the attribute, in each of the groups. (OCEBM Table of Evidence Glossary)

Case Series A group or series of case reports involving patients who were given similar treatment. Reports of case series usually contain detailed information about the individual patients. This includes demographic information (for example, age, gender, ethnic origin) and information on diagnosis, treatment, response to treatment, and follow-up after treatment. (OCEBM Table of Evidence Glossary)

Case Study An investigation of a single subject or a single unit, which could be a small number of individuals who seem to be representative of a larger group or very different from it. (Dictionary of Nursing Theory and Research, Fourth Edition)

Editorial Work consisting of a statement of the opinions, beliefs, and policy of the editor or publisher of a journal, usually on current matters of medical or scientific significance to the medical community or society at large. The editorials published by editors of journals representing the official organ of a society or organization are generally substantive. (PubMed)

Opinion A belief or conclusion held with confidence but not substantiated by positive knowledge or proof. (The Free Dictionary)

Animal Research A laboratory experiment using animals to study the development and progression of diseases. Animal studies also test how safe and effective new treatments are before they are tested in people.(NCI Dictionary of Cancer Terms)

In Vitro Research In the laboratory (outside the body). The opposite of in vivo (in the body). (NCI Dictionary of Cancer Terms)

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Climb the Evidence Pyramid for Better Research

Research evidence comes in various forms. The evidence pyramid is a hierarchical system of organizing and visualizing the different types of evidence.

The evidence pyramid visually shows the evidential strength of different types of research. All levels of the pyramid are “ valid ” evidence, but those at the higher levels are considered the most credible, for good reason.

At the very top are systematic reviews, which are scientific analyses of other scientific analysis.

What you’ll learn in this post

• What “levels of evidence” and the “hierarchy of evidence” are.

• Why certain types of evidence are seen as more valuable, but also why “lower-level” evidence can still be extremely useful.

• The evidence pyramid: a simple visual way of understanding the evidence hierarchy from bottom to top.

• Every level of evidence in detail, and its place in your research.

• Where to get expert research assistance and speed along your work to successful publication.

What is the evidence pyramid?

Evidence levels or categories were first theorized as a hierarchy in a 1979 Canadian report on health examinations . The authors developed a system of rating evidence to determine different interventions’ effectiveness and develop recommendations for periodic health exams.

The hierarchy of evidence is a key part of evidence-based medicine (EBM), a term coined in 1991 . From this term developed the related idea of evidence-based practice (EBP), in which all healthcare-related decisions should be based on the best available, up-to-date, and sound evidence.

The evidence pyramid, however, generally applies to all types of research. There are specific levels of evidence in different disciplines, like education, management, or economics. But the pyramid is primarily used in medicine and clinical studies .

A systematic review or meta-analysis in less than a month? You can do it.

Many scientific journals in the fields of medicine and health assign levels of evidence to studies based on their methodological quality. For example, since 2015, The Journal of Bone & Joint Surger y uses the hierarchical rating system proposed by the U.K. Centre for Evidence-Based Medicine (CEBM) .

Similarly, you might be asked to do this when submitting an abstract for a conference as an author. The evidence levels might differ slightly across fields. But they always aim to help readers know how valid your study’s results are and if they’re applicable to sound decision-making.

Unfiltered or Primary Literature

We’ll start from the bottom of the evidence pyramid and work our way to the top. While the highest level is the “best” (most desirable), lower levels all have a suitable place.

You may not always find information on your topic in the filtered literature, like if you’re researching a new topic, such as the impact of a new virus on perinatal fatality rates.

Move down to the next highest level of evidence that addresses your question when this happens. Your role is to review what you find to make sure it’s methodologically sound when researching databases with unfiltered information . You’d need to consider, for example, the quality of the journal in which the study is published.

Also, watch out for the nature of the peer review process (e.g., how many reviewers are involved).

Level 7: Expert opinions, editorials

Editorials and expert opinions are at the bottom of the pyramid because they’re not necessarily backed by robust evidence. They are truthful as much as any opinion can be, and they are by experts, but they’re not always reliable for statements of fact.

These are short articles on current issues of medical or scientific significance to the medical community or society at large. Authors aim to stimulate discussion and ideas around the development of evidence synthesis. They also try to promote sound policymaking.

Expert opinions

Among the properties of expert opinions is that they:

• Give the authors’ interpretation of the data presented in an article
• Discuss the research avenues that might emerge as we get more detailed results on the topic
• Offer a belief or conclusion held with confidence but not verified by positive knowledge or proof

Editorials and expert opinions are helpful if you’re new to a topic. They can give you general information that informs your research topic. Or they might help you identify potential keywords for your research.

However, it’s best not to take editorials and expert opinions at face value. When you’re reading one:

• Examine the author’s credentials
• Check if the sources cited (if any) are reliable and the most recent
• Check the endorsements and comments the editorial/expert opinion has received
• Check the publishing sources’ reputations

Level 6: Case-controlled studies, case series, and case reports

A case-controlled study is an observational study. Patients who already have a specific condition or disease (case group) are compared with people who don’t (control/comparison/reference group). These studies look back into time, and patient records are usually reviewed and analyzed.

For example, a 2018 case-controlled study examined the frequency of sleep disturbance in 179 patients with psoriasis compared with a control group of 105 people. Such studies require fewer resources than randomized controlled trials (Level 4) and cohort studies (Level 5), but are usually less reliable. That’s because showing a statistical relationship doesn’t mean that one factor necessarily caused the other.

For example, the case-controlled study cited above found twice as many patients with psoriasis reporting clinical insomnia compared with those from the control group. But the study does not exclude the possibility that other (social, psychological, or clinical) factors impacted the case group’s sleep patterns.

A case report is a report on the treatment and outcome of a single patient, while a case series is a collection (series) of case reports involving patients given similar treatment. They typically contain detailed information about the individual patients, such as:

• demographic information (e.g., age, gender, ethnic origin)
• information on diagnosis, treatment, response to treatment
• follow-up after treatment

These studies don’t use control groups with which to compare outcomes, so they have no statistical significance.

Level 5: Cohort studies

A cohort study works with a large population. It follows patients who have a specific condition or receive a particular treatment. And it compares them with another group that’s not affected by the condition or treatment. A cohort study’s central characteristic is its observing large numbers over a long time (usually years).

Cohort studies are beneficial if you look at a disease’s risk factors and causes. For example, you could examine whether there’s a cause and effect between non-health-related factors (e.g., women’s age or lifestyle choices, such as using oral contraceptives) and ovarian cancer.

Remember, though, that cohort studies have a high potential for bias (the tendency to influence a study’s results or interpretation). The two groups may differ in ways other than in the variable under investigation.

Level 4: Randomized control trials (RCTs)

Suppose you are a nurse practitioner wondering if you should suggest nicotine gum to help smokers quit smoking. You could look for evidence from randomized controlled trials (RCTs) to address this question.

In these rigorously planned experiments, subjects in a population (here smokers) are randomly (by chance) allocated to nicotine gum (study or intervention group) or placebo gum (control group). They are then followed up over time. Data are collected and compared with the number of participants in each group who quit smoking.

The random allocation of participants to groups and their longitudinal nature make RCTs methodologically robust. Generally, the RCT is the most rigorous design to determine whether some factor (exposure or intervention) causes an outcome, a study on evidence-based nursing suggests.

If your question is, “what will happen to this patient if we do nothing at all?”, RCTs might not be of help. This is because you’re looking at the likely course of a medical condition or disease that doesn’t involve comparing treatments. Here, the best evidence would come from a cohort study or a systematic review of cohort studies. 

The 12P Method for Systematic Reviews

We’ve squeezed all the steps and stages of a typical systematic review onto one page.

You can print it out A4-sized and use it as a handy checklist, or A3-sized for your laboratory wall. You can even share it with your co-authors.

FREE PDF CHECKLIST

Filtered or Secondary Literature

As you move up the pyramid, however, fewer studies exist. In the rest of this article, we’ll be looking at filtered literature that follows a stringent scientific methodology to appraise the quality of studies and make practical recommendations.

Level 3: Critically appraised individual articles

A critically appraised individual article (CAIA) identifies, evaluates, and synopsizes an individual study of excellent quality that will influence a standard of practice. It’s also known as an article synopsis.

Level 2: Critically appraised topics

After a CAIA, the next best source is a critically appraised topic (CAT), also known as Evidence Synthesis. A CAT summarizes the best available research evidence on a given topic or theme so that researchers and practitioners can use it to make informed decisions. Essentially, critically appraised topics are like shorter and less rigorous systematic reviews focused on a particular topic.

Keeping up with all the new evidence from your field is daunting and practically impossible. That’s where CAIAs and CATs come into play (along with systematic reviews). They help you make evidence-based decisions in research scenarios in a time-efficient and reliable way.

Level 1: Systematic reviews

However, systematic reviews are the gold standard in exploring, collating, assessing, and summarizing the best available evidence. Their primary aim is to recommend best practices on a specific topic and inform policy.

They follow a rigorous, standardized method (a pre-determined protocol) to identify, select, and evaluate all available peer-reviewed publications on the topic (e.g., poorly conducted studies are eliminated). Essentially, all eligible research studies are viewed as a “population” to be systematically sampled and investigated.

Systematic reviews summarize results according to preselected criteria and make practice recommendations based on methodologically sound studies. This method minimizes biases and random errors and explains why systematic reviews sit at the top of the pyramid.

A systematic review may include a meta-analysis. Meta-analyses use quantitative statistical methodology to combine the results of several credible studies drawn from the literature. Because of its quantitative methods, it’s a higher-quality systematic review. Its conclusions may be used to evaluate therapeutic effectiveness, plan new studies, etc.

When to use a systematic review

Systematic reviews come in handy if you want to recommend new approaches to practice. Also, when/if there are discrepancies in how a certain practice is performed. Let’s look at a relevant example from the plastic surgery literature.

An association was hypothesized between lymphoma among women with silicone breast implants . But findings of relevant case reports and cohort studies conducted were mixed. A systematic review combined all available evidence and reached a definite conclusion: there’s no credible evidence of an increase of lymphoma originating in the breast among women with cosmetic breast implants.

As a researcher, you’ll rarely get the time to track down and peruse all primary literature related to a specific topic of interest. A systematic review is your best source of evidence if you need to quickly and accurately keep up to date with evidence accumulating in your field.

More than one study usually addresses questions about the prevention or treatment of disease and the cause or course of a disease. Suppose you need top-level evidence on the effectiveness of different health interventions in addressing specific conditions . In this case, it’s helpful to turn to a systematic review summarizing the results of randomized controlled trials.

A systematic review in a month or less? You can do it. We can help.

Our published experts ready will assist you with all steps of the systematic review and meta-analysis processes. Go here to find how you can put Edanz on your team and, if you’re in a hurry, get your work one in just a few weeks.

Or see how Edanz’s other research services can help you reach new heights .

Evidence-Based Medicine: Types of Studies

• What is Evidence-Based Practice?
• Question Types and Corresponding Resources
• Types of Studies
• Practice Guidelines
• Step 3: Appraise This link opens in a new window
• Steps 4-5: Apply & Assess

Experimental vs. Observational Studies

An observational study is a study in which the investigator cannot control the assignment of treatment to subjects because the participants or conditions are not directly assigned by the researcher.

• Examines predetermined treatments, interventions, policies, and their effects
• Four main types: case series , case-control studies , cross-sectional studies , and cohort studies

In an experimental study , the investigators directly manipulate or assign participants to different interventions or environments

Experimental studies that involve humans are called clinical trials . They fall into two categories: those with controls, and those without controls.

• Controlled trials - studies in which the experimental drug or procedure is compared with another drug or procedure
• Uncontrolled trials - studies in which the investigators' experience with the experimental drug or procedure is described, but the treatment is not compared with another treatment

Definitions taken from: Dawson B, Trapp R.G. (2004). Chapter 2. Study Designs in Medical Research. In Dawson B, Trapp R.G. (Eds), Basic & Clinical Biostatistics, 4e . Retrieved September 15, 2014 from  https://accessmedicine.mhmedical.com/book.aspx?bookid=2724

Levels of Evidence Pyramid

Levels of Evidence Pyramid created by Andy Puro, September 2014

Additional Study Design Resources

Study Design 101 : Himmelfarb's tutorial on study types and how to find them

Study Designs  (Centre for Evidence Based Medicine, University of Oxford)

Learn about Clinical Studies  (ClinicalTrials.gov, National Institutes of Health)

Study Designs Guide  (Deakin University)

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Health Literacy: Evidence Pyramid

• Getting Started
• Popular vs. Academic Sources
• More Reputable Sources
• UNH Resources

Evidence Pyramid

Adaptation of the Evidence Pyramid Diagram developed by the Medical Research Library of Brooklyn, SUNY Downstate Medical Center.

Additional Learning Resources

• Overview of epidemiologic study designs From Essentials of Epidemiology in Public Health by Ann Aschengrau, Sc.D. and George R. Seage, III, Sc.D. Published by Jones and Bartlett (2003)
• BMJ (British Medical Journal): How to read a paper Links to articles in the BMJ that explain how to read and interpret different kinds of research papers.

Types of Research Studies

Systematic Review: a comprehensive summary of high-quality studies examining a given topic. Example: Will emergency and surgical patients participate in and complete alcohol interventions? A systematic review

Meta-Analysis: a type of systematic review where results from available high-quality studies are statistically combined to compute a net overall effect. Example: Alcohol drinking and bladder cancer risk: a meta-analysis

Randomized Controlled Trial: a type of epidemiologic study where participants are randomly assigned to receive a given exposure (such as a new drug or therapy) and then followed to examine the effects of the exposure on outcomes. Example: Treating alcohol withdrawal with oral baclofen: A randomized, double-blind, placebo-controlled trial

Cohort: a type pf epidemiologic study design where one or more population groups (called cohorts) are classified according to their level of exposure to a given agent/risk factor and followed over time to determine if this exposure is related to the occurrence of a disease or outcome of interest. Example: Effect of retirement on alcohol consumption: longitudinal evidence from the French Gazel cohort study

Case Control: a type of epidemiologic study that compares individuals who have a disease or outcome of interest (cases) with those who do not (controls). Researchers look retrospectively to evaluate how frequently exposure to a risk factor/agent is present in each group to identify the relationship between the risk factor and the disease or outcome of interest. Example: Risk factors for alcohol dependence: A case-control study

Cross-Sectional: a type of epidemiologic study that observes the relationship between a characteristic/risk factor (the exposure) and the prevalence of the disease or outcome of interest in a specific population at a single point in time. Example: Alcohol involvement in aggression between intimate partners in New Zealand: a national cross-sectional study

Case Series: a summary of a small group of individuals' experience with a similar disease or outcome of interest. Example: Alcohol use in chronic fatigue syndrome

Case Reports: a summary of one individual's experience with the disease or outcome of interest. Example: Acute coronary ischemia during alcohol withdrawal: a case report

Ideas, editorials, opinions: put forth by experts in the field Example: Alcohol brewing and the African tuberculosis epidemic

Animal Research Studies: studies conducted using animal subjects. Example: Renal effects of alcohol withdrawal in five-week alcohol-treated rats

Test-tube lab research: "test tube" experiments conducted in a controlled laboratory setting

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• Last Updated: Apr 20, 2022 12:55 PM
• URL: https://libraryguides.unh.edu/health-literacy

• Research Process

Levels of evidence in research

• 5 minute read
• 97.9K views

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

Find more about Levels of evidence in research on Pinterest:

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The Evidence-Based Medicine Pyramid!

Posted on 29th April 2014 by Danny Minkow

You have seen the Pyramids of Egypt, you’ve heard of Maslow’s Pyramid of Needs and you’ve gotten more than your fill of the Food Pyramid.

Now, behold! The wonders of the Evidence-Based Medicine Pyramid!

What is the Evidence-Based Medicine Pyramid and why is it important?

The Evidence-Based Medicine Pyramid is simply a diagram that was created to help us understand how to weigh different levels of evidence in order to make health-related decisions. It helps us put the results of each study design into perspective, based on the relative strengths and weaknesses of each design.

Allow me to be your guide as we tour the Evidence-Based Pyramid.

As we approach, we can see the Evidence-Based Pyramid is divided into levels. Each ascending level represents a different type of study design and corresponds to increasing rigor, quality, and reliability of the evidence.  In other words, as we ascend through these different study designs, we become more confident that their results are accurate, have less chance of statistical error, and minimize bias from confounding variables that could have influenced the results.

Information and expert opinion

The first level of the Evidence-Based Pyramid is the foundation. This background information is important and helpful. However, when viewed on its own, this type of evidence can be heavily influenced by beliefs, opinions, or even politics. This level might also include anecdotal evidence.

Case control studies or case series reports

This level represents the first stage of testing an observation. Case Series reports usually include only a few participants who are given a similar intervention and follow-up.   Case Control Studies are similar to case series, except it looks retrospectively at individuals and compares with a similar group who did not have the intervention.  These studies are conducted in the early stages of research to help identify variables that might predict a condition.  One of the weaknesses in these designs is that there are small numbers of participants and they are frequently not randomized or controlled for confounding variables.

Cohort studies

Also called longitudinal or epidemiological studies, Cohort studies follow a large group of people over an extended period of time to see how their exposures affect their outcomes. This type of study is normally used to look at the effect of suspected risk factors that cannot be controlled experimentally – for example, the effect of smoking on lung cancer. These studies are frequently used to determine long term effects of a lifestyle, diet, or other interventions. Cohort studies may include a second group that did not engage in the same intervention as a control comparison.  Although these studies are a step up in reliability and generalizability, they can be difficult to blind, can’t be controlled for outside variables, and are usually not randomized.

The Randomized Control Trial (RCT)

Now we have reached a major point in the Pyramid – the Randomized Control Trial, the true experimental design. In this study design, individuals are assigned by special randomization techniques into two or more groups, where one group receives the intervention under investigation and the other(s) receives no treatment, a placebo, or a standard intervention.

A large Double Blinded Randomized Control Trial is the most reliable “test” or study design and provides the strongest support of a cause and effect relationship. However, these studies are expensive and can be ethically problematic.

Critically appraised topics

Critically appraised topics are not actually a study design. They are short summaries of the best available evidence. Basically, these are an abbreviated systematic review created to answer a specific question.

The systematic reviews

Now we have finally arrived at the pinnacle of the Evidence-Based Medicine Pyramid.

From this dizzying height, we take in a panoramic view of all of the evidence about an intervention.   Systematic reviews take a bird’s eye view by comparing the results of studies side by side, typically on a forest plot. Systematic reviews are considered the strongest and highest quality of evidence.

We could also include m eta-analysis here as well. This is where multiple studies are reviewed and a statistical summary is made that represents the effect of the intervention across multiple studies.

Cochrane takes systematic reviews to the next level.  They are the experts of the systematic review and have an added a level of rigor as an independent voice, as well as developing special techniques to identify bias in studies.

Well, I hope you have enjoyed this tour of the Evidence-Based Pyramid. So next time you hear about a “study” in the media or anywhere else be sure to find out what kind of study design was and where it falls on the Evidence-Based Medicine Pyramid.

Useful links:

The pyramid was produced by HLWIKI Canada . The original link is unfortunately no longer available, but the image is available

Glover, J. (n.d.). Yale University Library Subject Guides. Evidence-Based Clinical Practice Resources. Pyramid . Retrieved April 10, 2014.

Sedlacek, B. (n.d.). JML LibGuides. Evidence-Based Practice. Evidence Pyramid/Study Designs . Retrieved April 14, 2014.

Danny Minkow

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Good overview

hope your research goes well!!!!

I am lookinf for the source of the pyramid, i.e., I want to cite it. Apparently, the HLWIKI does not exist anymore..

There are several variants on the internet (e.g., https://libguides.methodistcollege.edu/ebp/ebplevels , https://researchguides.library.tufts.edu/c.php?g=454103&p=3102416#s-lg-box-13868045 ). Some suggest that the pyramid is based on the Oxford Centre for Evidence-Based Medicine (CEBM) Levels of Evidence Table. But I cannot find a source that explicitely confirm so, or even shows how the pyramid was based on the table.

Any ideas who exactly made the pyramid, and based on what?

Hi Srinchtai. That is an interesting question and one that I don’t know the immediate answer to. As you have done, I have searched around for the original source now that the HLWIKI no longer exists and there are many sources using the pyramid. The Wikipedia page is interesting in mapping out the potential history: https://en.wikipedia.org/wiki/Hierarchy_of_evidence and one of our subsequent bloggers has written about the need to update the EBM period: https://s4be.cochrane.org/blog/2017/08/04/an-update-to-evidence-based-medicine-pyramid-why-and-how/ . I will endeavour to search some more and ask the question of the S4BE community too.

Hello to every body, it’s my first pay a visit of this website; this web site contains remarkable and really good data in favoir of readers.

Thank you — sure helps !

May I use your ‘evidence pyramid’ image for a class on information and research literacy for massage therapists?

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Levels of Evidence

• Study Design 101 From George Washington University. This webpage provides an alternative version of the evidence pyramid. Click each level of the pyramid to see more information about that study design, including a basic definition; pros, cons, and pitfalls of the design; and a glossary of study design terms.
• Evidence-Based Medicine Guide From Georgetown University. This guide provides another view of the evidence pyramid, including thorough descriptions of the different study designs and the types of questions they typically answer.
• Research Designs From University of Oxford - Centre for Evidence Based Intervention (CEBI). This webpage provides additional information about the different research designs that appear in the pyramid of evidence.

Question Type & Study Design

What type of question you are trying to answer (e.g. therapy, diagnosis, etc.) will directly affect what types of evidence you can expect or hope to find.  For example, a therapy question would ideally be answered by a randomized controlled trial.  However, it is not very ethical to randomize people to experience a potential harm, so an etiology question may be more appropriately answered by a cohort or case-control study.

When assessing the evidence you are finding, you want to aim for the top of the pyramid, but also consider if that evidence would exist and what type of study would best encompass it.

Is this Scholarly?

The research evidence integrated in Evidence-Based Practice should come from scholarly sources.  If you are unsure whether you are using a scholarly article, the following resources can help.

Video created by University of Washington Libraries.  This video is licensed under a Creative Commons Attribution 4.0 International License .

• Distinguishing Scholarly From Non-Scholarly Periodicals Guide This guide from Cornell University provides an overview of the different types of sources you may come across in your research.
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1.9: Types of Research Studies and How To Interpret Them

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• Alice Callahan, Heather Leonard, & Tamberly Powell
• Lane Community College via OpenOregon

The field of nutrition is dynamic, and our understanding and practices are always evolving. Nutrition scientists are continuously conducting new research and publishing their findings in peer-reviewed journals. This adds to scientific knowledge, but it’s also of great interest to the public, so nutrition research often shows up in the news and other media sources. You might be interested in nutrition research to inform your own eating habits, or if you work in a health profession, so that you can give evidence-based advice to others. Making sense of science requires that you understand the types of research studies used and their limitations.

The Hierarchy of Nutrition Evidence

Researchers use many different types of study designs depending on the question they are trying to answer, as well as factors such as time, funding, and ethical considerations. The study design affects how we interpret the results and the strength of the evidence as it relates to real-life nutrition decisions. It can be helpful to think about the types of studies within a pyramid representing a hierarchy of evidence, where  the  studies at the bottom of the pyramid usually give us the weakest evidence with the least relevance to real-life nutrition decisions, and the studies at the top offer the strongest evidence, with the most relevance to real-life nutrition  decisions .

The pyramid also represents a few other general ideas. There tend to be more studies published using the methods at the bottom of the pyramid, because they require less time, money, and other resources. When researchers want to test a new hypothesis , they often start with the study designs at the bottom of the pyramid , such as in vitro, animal, or observational studies. Intervention studies are more expensive and resource-intensive, so there are fewer of these types of studies conducted. But they also give us higher quality evidence, so they’re an important next step if observational and non-human studies have shown promising results. Meta-analyses and systematic reviews combine the results of many studies already conducted, so they help researchers summarize scientific knowledge on a topic.

Non-Human Studies: In Vitro & Animal Studies

The simplest form of nutrition research is an in vitro study . In vitro means “within glass,” (although plastic is used more commonly today) and these experiments are conducted within flasks, dishes, plates, and test tubes. These studies are performed on isolated cells or tissue samples, so they’re less expensive and time-intensive than animal or human studies. In vitro studies are vital for zooming in on biological mechanisms, to see how things work at the cellular or molecular level. However, these studies shouldn’t be used to draw conclusions about how things work in humans (or even animals), because we can’t assume that the results will apply to a whole, living organism.

Animal studies are one form of in vivo research, which translates to “within the living.” Rats and mice are the most common animals used in nutrition research. Animals are often used in research that would be unethical to conduct in humans. Another advantage of animal dietary studies is that researchers can control exactly what the animals eat. In human studies, researchers can tell subjects what to eat and even provide them with the food, but they may not stick to the planned diet. People are also not very good at estimating, recording, or reporting what they eat and in what quantities. In addition, animal studies typically do not cost as much as human studies.

There are some important limitations of animal research. First, an animal’s metabolism and physiology are different from humans. Plus, animal models of disease (cancer, cardiovascular disease, etc.), although similar, are different from human diseases. Animal research is considered preliminary, and while it can be very important to the process of building scientific understanding and informing the types of studies that should be conducted in humans, animal studies shouldn’t be considered relevant to real-life decisions about how people eat.

Observational Studies

Observational studies  in human nutrition collect information on people’s dietary patterns or nutrient intake and look for associations with health outcomes. Observational studies do not give participants a treatment or intervention; instead, they look at what they’re already doing and see how it relates to their health. These types of study designs can only identify  correlations  (relationships) between nutrition and health; they can’t show that one factor  causes  another. (For that, we need intervention studies, which we’ll discuss in a moment.) Observational studies that describe factors correlated with human health are also called  epidemiological studies . 1

One example of a nutrition hypothesis that has been investigated using observational studies is that eating a Mediterranean diet reduces the risk of developing cardiovascular disease. (A Mediterranean diet focuses on whole grains, fruits and vegetables, beans and other legumes, nuts, olive oil, herbs, and spices. It includes small amounts of animal protein (mostly fish), dairy, and red wine. 2 ) There are three main types of observational studies, all of which could be used to test hypotheses about the Mediterranean diet:

• Cohort studies follow a group of people (a cohort) over time, measuring factors such as diet and health outcomes. A cohort study of the Mediterranean diet would ask a group of people to describe their diet, and then researchers would track them over time to see if those eating a Mediterranean diet had a lower incidence of cardiovascular disease.
• Case-control studies compare a group of cases and controls, looking for differences between the two groups that might explain their different health outcomes. For example, researchers might compare a group of people with cardiovascular disease with a group of healthy controls to see whether there were more controls or cases that followed a Mediterranean diet.
• Cross-sectional studies collect information about a population of people at one point in time. For example, a cross-sectional study might compare the dietary patterns of people from different countries to see if diet correlates with the prevalence of cardiovascular disease in the different countries.

Prospective cohort studies, which enroll a cohort and follow them into the future, are usually considered the strongest type of observational study design. Retrospective studies look at what happened in the past, and they’re considered weaker because they rely on people’s memory of what they ate or how they felt in the past. There are several well-known examples of prospective cohort studies that have described important correlations between diet and disease:

• Framingham Heart Study : Beginning in 1948, this study has followed the residents of Framingham, Massachusetts to identify risk factors for heart disease.
• Health Professionals Follow-Up Study : This study started in 1986 and enrolled 51,529 male health professionals (dentists, pharmacists, optometrists, osteopathic physicians, podiatrists, and veterinarians), who complete diet questionnaires every 2 years.
• Nurses Health Studies : Beginning in 1976, these studies have enrolled three large cohorts of nurses with a total of 280,000 participants. Participants have completed detailed questionnaires about diet, other lifestyle factors (smoking and exercise, for example), and health outcomes.

Observational studies have the advantage of allowing researchers to study large groups of people in the real world, looking at the frequency and pattern of health outcomes and identifying factors that correlate with them. But even very large observational studies may not apply to the population as a whole. For example, the Health Professionals Follow-Up Study and the Nurses Health Studies include people with above-average knowledge of health. In many ways, this makes them ideal study subjects, because they may be more motivated to be part of the study and to fill out detailed questionnaires for years. However, the findings of these studies may not apply to people with less baseline knowledge of health.

We’ve already mentioned another important limitation of observational studies—that they can only determine correlation, not causation. A prospective cohort study that finds that people eating a Mediterranean diet have a lower incidence of heart disease can only show that the Mediterranean diet is correlated with lowered risk of heart disease. It can’t show that the Mediterranean diet directly prevents heart disease. Why? There are a huge number of factors that determine health outcomes such as heart disease, and other factors might explain a correlation found in an observational study. For example, people who eat a Mediterranean diet might also be the same kind of people who exercise more, sleep more, have higher income (fish and nuts can be expensive!), or be less stressed. These are called confounding factors ; they’re factors that can affect the outcome in question (i.e., heart disease) and also vary with the factor being studied (i.e., Mediterranean diet).

Intervention Studies

Intervention studies , also sometimes called experimental studies or clinical trials, include some type of treatment or change imposed by the researcher. Examples of interventions in nutrition research include asking participants to change their diet, take a supplement, or change the time of day that they eat. Unlike observational studies, intervention studies can provide evidence of cause and effect , so they are higher in the hierarchy of evidence pyramid.

The gold standard for intervention studies is the randomized controlled trial (RCT) . In an RCT, study subjects are recruited to participate in the study. They are then randomly assigned into one of at least two groups, one of which is a control group (this is what makes the study controlled ). In an RCT to study the effects of the Mediterranean diet on cardiovascular disease development, researchers might ask the control group to follow a low-fat diet (typically recommended for heart disease prevention) and the intervention group to eat a Mediterrean diet. The study would continue for a defined period of time (usually years to study an outcome like heart disease), at which point the researchers would analyze their data to see if more people in the control or Mediterranean diet had heart attacks or strokes. Because the treatment and control groups were randomly assigned, they should be alike in every other way except for diet, so differences in heart disease could be attributed to the diet. This eliminates the problem of confounding factors found in observational research, and it’s why RCTs can provide evidence of causation, not just correlation.

Imagine for a moment what would happen if the two groups weren’t randomly assigned. What if the researchers let study participants choose which diet they’d like to adopt for the study? They might, for whatever reason, end up with more overweight people who smoke and have high blood pressure in the low-fat diet group, and more people who exercised regularly and had already been eating lots of olive oil and nuts for years in the Mediterranean diet group. If they found that the Mediterranean diet group had fewer heart attacks by the end of the study, they would have no way of knowing if this was because of the diet or because of the underlying differences in the groups. In other words, without randomization, their results would be compromised by confounding factors, with many of the same limitations as observational studies.

In an RCT of a supplement, the control group would receive a placebo—a  “fake” treatment that contains no active ingredients, such as a sugar pill. The use of a placebo is necessary in medical research because of a phenomenon known as the placebo effect. The placebo effect results in a beneficial effect because of a subject’s belief in the treatment, even though there is no treatment actually being administered.

Blinding is a technique to prevent bias in intervention studies. In a study without blinding, the subject and the researchers both know what treatment the subject is receiving. This can lead to bias if the subject or researcher have expectations about the treatment working, so these types of trials are used less frequently. It’s best if a study is double-blind , meaning that neither the researcher nor the subject know what treatment the subject is receiving. It’s relatively simple to double-blind a study where subjects are receiving a placebo or treatment pill, because they could be formulated to look and taste the same. In a single-blind study , either the researcher or the subject knows what treatment they’re receiving, but not both. Studies of diets—such as the Mediterranean diet example—often can’t be double-blinded because the study subjects know whether or not they’re eating a lot of olive oil and nuts. However, the researchers who are checking participants’ blood pressure or evaluating their medical records could be blinded to their treatment group, reducing the chance of bias.

Like all studies, RCTs and other intervention studies do have some limitations. They can be difficult to carry on for long periods of time and require that participants remain compliant with the intervention. They’re also costly and often have smaller sample sizes. Furthermore, it is unethical to study certain interventions. (An example of an unethical intervention would be to advise one group of pregnant mothers to drink alcohol to determine its effects on pregnancy outcomes, because we know that alcohol consumption during pregnancy damages the developing fetus.)

VIDEO: “ Not all scientific studies are created equal ” by David H. Schwartz, YouTube (April 28, 2014), 4:26.

Meta-Analyses and Systematic Reviews

At the top of the hierarchy of evidence pyramid are systematic reviews and meta-analyses .  You can think of these as “studies of studies.” They attempt to combine all of the relevant studies that have been conducted on a research question and summarize their overall conclusions. Researchers conducting a  systematic review  formulate a research question and then systematically and independently identify, select, evaluate, and synthesize all high-quality evidence that relates to the research question. Since systematic reviews combine the results of many studies, they help researchers produce more reliable findings. A  meta-analysis  is a type of systematic review that goes one step further, combining the data from multiple studies and using statistics to summarize it, as if creating a mega-study from many smaller studies . 4

However, even systematic reviews and meta-analyses aren’t the final word on scientific questions. For one thing, they’re only as good as the studies that they include. The  Cochrane Collaboration  is an international consortium of researchers who conduct systematic reviews in order to inform evidence-based healthcare, including nutrition, and their reviews are among the most well-regarded and rigorous in science. For the most recent Cochrane review of the Mediterranean diet and cardiovascular disease, two authors independently reviewed studies published on this question. Based on their inclusion criteria, 30 RCTs with a total of 12,461 participants were included in the final analysis. However, after evaluating and combining the data, the authors concluded that “despite the large number of included trials, there is still uncertainty regarding the effects of a Mediterranean‐style diet on cardiovascular disease occurrence and risk factors in people both with and without cardiovascular disease already.” Part of the reason for this uncertainty is that different trials found different results, and the quality of the studies was low to moderate. Some had problems with their randomization procedures, for example, and others were judged to have unreliable data. That doesn’t make them useless, but it adds to the uncertainty about this question, and uncertainty pushes the field forward towards more and better studies. The Cochrane review authors noted that they found seven ongoing trials of the Mediterranean diet, so we can hope that they’ll add more clarity to this question in the future. 5

Science is an ongoing process. It’s often a slow process, and it contains a lot of uncertainty, but it’s our best method of building knowledge of how the world and human life works. Many different types of studies can contribute to scientific knowledge. None are perfect—all have limitations—and a single study is never the final word on a scientific question. Part of what advances science is that researchers are constantly checking each other’s work, asking how it can be improved and what new questions it raises.

Attributions:

• “Chapter 1: The Basics” from Lindshield, B. L. Kansas State University Human Nutrition (FNDH 400) Flexbook. goo.gl/vOAnR , CC BY-NC-SA 4.0
• “ The Broad Role of Nutritional Science ,” section 1.3 from the book An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0

References:

• 1 Thiese, M. S. (2014). Observational and interventional study design types; an overview. Biochemia Medica , 24 (2), 199–210. https://doi.org/10.11613/BM.2014.022
• 2 Harvard T.H. Chan School of Public Health. (2018, January 16). Diet Review: Mediterranean Diet . The Nutrition Source. https://www.hsph.harvard.edu/nutritionsource/healthy-weight/diet-reviews/mediterranean-diet/
• 3 Ross, R., Gray, C. M., & Gill, J. M. R. (2015). Effects of an Injected Placebo on Endurance Running Performance. Medicine and Science in Sports and Exercise , 47 (8), 1672–1681. https://doi.org/10.1249/MSS.0000000000000584
• 4 Hooper, A. (n.d.). LibGuides: Systematic Review Resources: Systematic Reviews vs Other Types of Reviews . Retrieved February 7, 2020, from //libguides.sph.uth.tmc.edu/c.php?g=543382&p=5370369
• 5 Rees, K., Takeda, A., Martin, N., Ellis, L., Wijesekara, D., Vepa, A., Das, A., Hartley, L., & Stranges, S. (2019). Mediterranean‐style diet for the primary and secondary prevention of cardiovascular disease. Cochrane Database of Systematic Reviews , 3 . doi.org/10.1002/14651858.CD009825.pub3
• Figure 2.3. The hierarchy of evidence by Alice Callahan, is licensed under CC BY 4.0
• Research lab photo by National Cancer Institute on Unsplas h ; mouse photo by vaun0815 on Unsplash
• Figure 2.4. “Placebo effect example” by Lindshield, B. L. Kansas State University Human Nutrition (FNDH 400) Flexbook. goo.gl/vOAnR

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Observational Study Designs: Synopsis for Selecting an Appropriate Study Design

Assad a rezigalla.

1 Department of Basic Medical Sciences, College of Medicine, University of Bisha, Bisha, SAU

The selection of a study design is the most critical step in the research methodology. Crucial factors should be considered during the selection of the study design, which is the formulated research question, as well as the method of participant selection. Different study designs can be applied to the same research question(s). Research designs are classified as qualitative, quantitative, and mixed design. Observational design occupies the middle and lower parts of the hierarchy of evidence-based pyramid. The observational design is subdivided into descriptive, including cross-sectional, case report or case series, and correlational, and analytic which includes cross-section, case-control, and cohort studies. Each research design has its uses and points of strength and limitations. The aim of this article to provide a simplified approach for the selection of descriptive study design.

Introduction and background

A research design is defined as the “set up to decide on, among other issues, how to collect further data, analyze and interpret them, and finally, to provide an answer to the question” [ 1 ]. The primary objective of a research design is to guarantee that the collected evidence allows the answering of the initial question(s) as clearly as possible [ 2 ]. Various study designs have been described in the literature [ 1 - 3 ]. Each of them deals with the specific type of research or research questions and has points of strength and weakness. Broadly, research designs are classified into qualitative and quantitative research and mixed methods [ 3 ]. The quantitative study design is subdivided into descriptive versus analytical study designs or as observational versus interventional (Figure ​ (Figure1). 1 ). Descriptive designs occupy the middle and lower parts of the hierarchy of evidence-based medicine pyramid. Study designs are organized in a hierarchy beginning from the basic "case report" to the highly valued "randomised clinical trial" [ 4 - 5 ].

Case report

The case report describes an individual case or cases in their natural settings. Also, it describes unrecognized syndromes or variants, abnormal findings or outcomes, or association between risk factors and disease. It is the lowest level and the first line of evidence and usually deals with the newly emerging issues and ideas (Table ​ (Table1) 1 ) [ 4 , 6 - 10 ].

Case series

A case series is a report on data from a subject group (multiple patients) without control [ 6 , 11 - 12 ]. Commonly, this design is used for the illustration of novel, unusual, or atypical features identified in medical practice [ 6 ]. The investigator is governed by the availability and accuracy of the records, which can cause biases [ 13 - 14 ]. Bias in a case series can be decreased through consecutive patient enrollment and predefined inclusion and exclusion criteria, explicit specification of study duration, and enrollment of participants (Table 2 ) [ 11 - 12 ].

Correlational study design

Correlational studies (ecologic studies) explore the statistical relationships between the outcome of interest in population and estimate the exposures. It deals with the community rather than in individual cases. The correlational study design can compare two or more relevant variables and reports the association between them without controlling the variables. The aim of correlational study design or research is to uncover any types of systematic relationships between the studied variables. Ecological studies are often used to measure the prevalence and incidence of disease, mainly when the disease is rare. The populations compared can be defined in several ways, such as geographical, time trends, migrants, longitudinal, occupation, and social class. It should be considered that in ecological studies, the results are presented at the population (group) level rather than individuals. Ecological studies do not provide information about the degree or extent of exposure or outcome of interest for particular individuals within the study group (Table  3 ) [ 7 ,  15 - 16 ]. For example, we do not know whether those individuals who died in the study group under observation had higher exposure than those remained alive.

Cross-sectional study design

The cross-sectional study examines the association between exposures and outcomes on a snap of time. The assessed associations are guided by sound hypotheses and seen as hypothesis-generating [ 17 ]. This design can be descriptive (when dealing with prevalence or survey) or analytic (when comparing groups) [ 17 - 18 ]. The selection of participants in a cross-sectional study design depends on the predefined inclusion and exclusion criteria [ 18 - 19 ]. This method of selection limits randomization (Table 4 ).

Case-control study

A case-control study is an observational analytic retrospective study design [ 12 ]. It starts with the outcome of interest (referred to as cases) and looks back in time for exposures that likely caused the outcome of interest [ 13 , 20 ]. This design compares two groups of participants - those with the outcome of interest and the matched control [ 12 ]. The controls should match the group of interest in most of the aspects, except for the outcome of interest [ 18 ]. The controls should be selected from the same localization or setting of the cases [ 13 , 21 - 22 ]. Case-control studies can determine the relative importance of a predictor variable about the presence or absence of the disease (Table ​ (Table5 5 ).

Cohort study design

The cohort study design is classified as an observational analytic study design. This design compares two groups, with exposure of interest and control one [ 12 , 18 , 22 - 24 ].

Cohort design starts with exposure of interest comparing them to non-exposed participants at the time of study initiation [ 18 , 22 , 24 ]. The non-exposed serve as external control. A cohort design can be either prospective [ 18 ] or retrospective [ 12 , 20 , 24 - 25 ]. In prospective cohort studies, the investigator measures a variety of variables that might be a risk factor or relevant to the development of the outcome of interest. Over time, the participants are observed to detect whether they develop the outcome of interest or not. In this case, the participants who do not develop the outcome of interest can act as internal controls. Retrospective cohort studies use data records that were documented for other purposes. The study duration may vary according to the commencement of data recording. Completion of the study is limited to the analysis of the data [ 18 , 22 , 24 ]. In 2016, Setia reported that, in some instances, cohort design could not be well-defined as prospective or retrospective; this happened when retrospective and prospective data were collected from the same participants (Table ​ (Table6) 6 ) [ 24 ].

The selection of the study design is the most critical step in research methodology [ 4 , 26 ]. An appropriate study design guarantees the achievement of the research objectives. The crucial factors that should be considered in the selection of the study design are the formulated research question, as well as the method of sampling [ 4 , 27 ]. The study design determines the way of sampling and data analysis [ 4 ]. The selection of a research study design depends on many factors. Two crucial points that should be noted during the process selection include different study designs that may be applicable for the same research question(s) and researches may have grey areas in which they have different views about the type of study design [ 4 ].

Conclusions

The selection of appropriate study designs for research is critical. Many research designs can apply to the same research. Appropriate selection guarantees that the author will achieve the research objectives and address the research questions.

Acknowledgments

The author would like to acknowledge Dr. M. Abass, Dr. I. Eljack, Dr. K. Salih, Dr. I. Jack, and my colleagues. Special thanks and appreciation to the college dean and administration of the College of Medicine, University of Bisha (Bisha, Saudi Arabia) for help and allowing the use of facilities.

The content published in Cureus is the result of clinical experience and/or research by independent individuals or organizations. Cureus is not responsible for the scientific accuracy or reliability of data or conclusions published herein. All content published within Cureus is intended only for educational, research and reference purposes. Additionally, articles published within Cureus should not be deemed a suitable substitute for the advice of a qualified health care professional. Do not disregard or avoid professional medical advice due to content published within Cureus.

The authors have declared that no competing interests exist.

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November 7, 2023

Intermittent fasting for weight loss in people with type 2 diabetes

At a glance.

• People with obesity and type 2 diabetes lost more weight using daily periods of fasting than by trying to restrict calories over a six-month period.
• Blood sugar levels lowered in people in both groups, and no serious side effects were observed.

Around 1 in 10 Americans live with type 2 diabetes, a disease in which levels of blood glucose, or blood sugar, are too high. Diabetes can lead to serious health issues such as heart disease, nerve damage, and eye problems.

Excess weight is a major risk factor for the development of type 2 diabetes, and weight loss is often recommended for those with excess weight and type 2 diabetes. Calorie restriction—reducing overall calorie intake—is a mainstay of most weight loss programs. But such regimens are very difficult to stick with over the long term.

Time-restricted eating, also called intermittent fasting, has emerged as an alternative weight loss paradigm. In this approach, the time of day during which food can be eaten is restricted, but the amount or types of food are not. Small studies have suggested that intermittent fasting is safe and promotes weight loss in people with type 2 diabetes. But these studies only tracked participants for a short period of time. They also didn’t compare the approach with traditional calorie restriction.

In a new clinical trial, an NIH-funded research team led by Dr. Krista Varady from the University of Illinois Chicago compared fasting and calorie restriction for weight loss and blood-sugar reduction. They recruited 75 people with obesity and type 2 diabetes. Of these, 70 were either Hispanic or non-Hispanic Black—two groups in the U.S. with an especially high prevalence of diabetes. The participants were randomly assigned to one of three diet groups for six months.

The fasting group could eat anything they wanted, but only between the hours of noon and 8 pm. The second group worked with a dietitian to reduce their calories by 25% of the amount needed to maintain their weight. A control group did not change their diet at all. All groups received education on healthy food choices and monitored their blood glucose closely during the study. The results were published on October 27, 2023, in JAMA Network Open .

After six months, participants in the fasting group lost an average of 3.6% percent of their body weight compared to those in the control group. In comparison, people in the calorie-restriction group did not lose a significant amount of weight compared to the control group.

Both groups had similarly healthy decreases in their average blood glucose levels. Both also had reductions in waist circumference. No serious side effects, including time outside of a safe blood glucose range, were seen in either treatment group. People in the fasting group reported that their diet was easier to adhere to than calorie restriction.

“Our study shows that time-restricted eating might be an effective alternative to traditional dieting for people who can’t do the traditional diet or are burned out on it,” Varady says. “For many people trying to lose weight, counting time is easier than counting calories.”

Some medications used to treat type 2 diabetes need adjustment for time-restricted eating. Therefore, people considering intermittent fasting should speak with a doctor before changing their eating pattern.

—by Sharon Reynolds

Related Links

• Research in Context: Obesity and Metabolic Health
• Calorie Restriction and Human Muscle Function
• Popular Diabetes Drugs Compared in Large Trial
• Diabetes Control Worsened Over the Past Decade
• Fasting Increases Health and Lifespan in Male Mice
• Factors Contributing to Higher Incidence of Diabetes for Black Americans
• Diabetes Increasing in Youths
• Benefits of Moderate Weight Loss in People with Obesity
• To Fast or Not to Fast: Does When You Eat Matter?
• Managing Diabetes: New Technologies Can Make It Easier
• Type 2 Diabetes

References:  Effect of Time-Restricted Eating on Weight Loss in Adults With Type 2 Diabetes: A Randomized Clinical Trial. Pavlou V, Cienfuegos S, Lin S, Ezpeleta M, Ready K, Corapi S, Wu J, Lopez J, Gabel K, Tussing-Humphreys L, Oddo VM, Alexandria SJ, Sanchez J, Unterman T, Chow LS, Vidmar AP, Varady KA . JAMA Netw Open . 2023 Oct 2;6(10):e2339337. doi: 10.1001/jamanetworkopen.2023.39337. PMID: 37889487.

Funding:  NIH’s National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK); University of Illinois.

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