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Hierarchy of Evidence
The hierarchy of evidence is a core principle of Evidence-Based Practice (EBP). It allows you to answer the critical question of “what is the best available evidence that allows you to make a clinical decision”?
The evidence hierarchy allows you to take a top-down approach to locating the best evidence whereby you first search for a recent well-conducted systematic review and if that is not available, then move down to the next level of evidence to answer your question.
EBP hierarchies rank study types based on the strength and precision of their research methods.
Most experts agree that the higher up the hierarchy the study design is positioned, the more rigorous the methodology and hence the more likely it is that the study design can minimize the effect of bias on the results of the study. In most evidence hierarchies current, well designed systematic reviews and meta-analyses are at the top of the pyramid, and expert opinion and anecdotal experience are at the bottom. [1] [2][5]
The Evidence Based Medicine Pyramid presented below is a widely accepted model:
Understanding the Evidence Pyramid [1][3][4][5]
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
Top of the Pyramid - Filtered Evidence
Meta Analyses and Systematic Reviews [3] [4] [5]
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 mentioned above and limit your search to systematic review and meta-analysis.
The best bet for systematic reviews is the Cochrane Database for medical and social science topics. The Cochrane through Wiley is the best interface even though it is not full text.
The Cochrane Database of Systematic Reviews (CDSR)* Consists of detailed, structured topic reviews of hundreds of articles. Teams of experts complete comprehensive literature reviews, evaluate the literature, and present summaries of the findings of the best studies. Published by the International Cochrane Collaboration.
Systematic Reviews are also searchable in these sources:
CINAHL Complete *: Enter your search query. Select “Systematic Reviews” under “Publication Type”.
PubMed Clinical Queries: Enter your search query in the search box and click search. The Systematic Reviews search results are shown in the Center Column. Move the page down and select See all.
Critically Appraised Topic (CAT)[3] [4] [5]
A critically appraised topic 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:
Critically-Appraised Individual Article (CAIAs)[3] [4] [5]
A critically-appraised individual article (CAIAs) identifies, evaluates and synopsizes 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:
The TRIP (Turning Research Into Practice) Database simultaneously searches evidence-based sources of systematic reviews, practice guidelines, and critically-appraised topics and articles. Also searches MEDLINE’s Clinical Queries, medical image databases, e-textbooks, and patient information leaflets.
Base of the Pyramid - Unfiltered Evidence [3] [4] [5]
Base of the Pyramid - Unfiltered Evidence [3] [4] [5]
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.
Unfiltered evidence can be found in the following databases:
[2] Hoffman, T., Bennett, S., & Del Mar, C. (2013). Evidence-Based Practice: Across the Health Professions (2nd ed.). Chatswood, NSW: Elsevier.
[3] Evidence-Based Practice | Harvey Cushing/John Hay Whitney Medical Library (yale.edu)
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