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Evidence Based Medicine: The Evidence Hierarchy

The Evidence Hierarchy

The hierarchy of evidence is a core principal of EBM. EBM hierarchies rank study types based on the strength and precision of their research methods. Different hierarchies exist for different question types, and even experts may disagree on the exact rank of information in the evidence hierarchies. Still, most agree that current, well designed systematic reviews and meta-analyses are at the top of the pyramid, and that expert opinion and anecdotal experience are at the bottom.

The Evidence Hierarchy, Questions about Treatment

EBM hierarchy of evidence pyramid adapted from figure 2.1 in Greenhalgh How to Read a Paper 2010 ISBN: 978-1444390360

Figure adapted from Greenhalgh T. How to Read a Paper: The Basics of Evidence-Based Medicine. Hoboken: John Wiley & Sons, Incorporated; 2010. Figure 2.1 A simple hierarchy of evidence for assessing the quality of trial design in therapy studies.

Primary Studies

If a current, well designed systematic review is not available, go to primary studies to answer your question. The best research designs for a primary study varies depending on the question type.

The table below lists optimal study methodologies for the five types of questions considered in this tutorial, and is based on guidelines from the Centre for Health Evidence (PDF) and the ACP Journal Club.

Question Type

Optimal Study Methodology

Treatment (Therapy) & Prevention

  • Randomized controlled trial (RCT)


  • Randomized controlled trial (RCT)
  • Crossover study: subjects receive both the intervention (diagnostic test) under investigation and the reference standard or "gold standard" test

(Natural History)

  • Cohort study

Etiology or Harm

  • Randomized controlled trial (RCT), or, if RCTs are impractical or unethical, Cohort study or Case control study


The evidence hierarchy provides a clear strategy for your search of the primary literature: look first for reports of clinical trials that used the best research designs. Remember as you search, though, that the best available evidence may not come from the optimal study type. For example, if treatment effects found in well designed cohort studies are sufficiently large and consistent, those cohort studies may provide more compelling evidence than the findings from a weaker RCT. And, of course, the strongest research evidence does not exist for every question: it is difficult to blind studies of surgical treatments and ethical standards prohibit RCTs that would expose subjects to harm.

Evidence Syntheses: Systematic Reviews and Meta-Analysis

Most experts consider well done systematic reviews, with or without meta-analysis, to provide the best evidence for all question types. The major advantage of systematic reviews is that they are based on the findings of multiple studies that were identified in comprehensive, systematic literature searches. 

The position of systematic reviews at the top of the evidence hierarchy is not absolute, however. 

  • You must appraise a systematic review's methodological rigor and the strength of its findings before applying those findings to patient care.
  • Systematic reviews often take months to years to conduct. Make sure that the findings of a systematic review have not been superseded by newer evidence.
  • A large, well conducted randomized controlled trial may provide more compelling evidence than a systematic review of small, underpowered trials.

Systematic Reviews vs. Narrative Reviews

It is important to understand the difference between systematic reviews (with or without meta analysis) and narrative reviews.

Systematic reviews seek to answer a specific and clearly formulated question by using rigorous, explicit protocols to identify, select and appraise relevant research studies; and to collect and analyze data from the selected studies. To minimize bias, systematic reviews include or exclude evidence on the basis of explicit quality criteria.They may incorporate meta-analysis.

Narrative reviews, often just called Reviews, articles may be evidence-based, but they are not evidence. Rather than answering a specific clinical question, they provide an overview of the research landscape on a given topic. They usually lack systematic search protocols or explicit criteria for selecting and appraising evidence. 

Systematic Reviews
Narrative Reviews
Investigate a clearly defined, often very specific topic or question. (e.g. To assess the efficiacy and safety of ustekinumab in patients with moderate to severe placque psoriasis.) Typically provides a broadstrokes view of research in given area. (e.g. New treatment options for psoriasis.)
Literature is gathered using explicit search protocols. Extensive searching may be conducted, but an explicit, systematic literature search protocol not typically used.
Studies selected for the review using a protocol that specifies inclusion, exclusion criteria. Studies used to support the reviewers' recommendations are not selected according to a set of predetermined inclusion/exclusion criteria.
A critical appraisal process takes place, examining the quality of each study and determining the validity of its results.  Studies may be graded according to an established set of criteria. Mayor may not use a level of evidence rating system to "grade" the quality and strength of individual studies.
Data from primary study may be synthesized in a meta-analysis. Narrative reviews typically do no included a quantitative aspect.
When evidence is lacking, the authors usually recommend further research. When evidence is lacking, the authors make recommendations based on their opinions and experience. They made grade the strength of the recommendations depending on the strength of the underlying evidence used to arrive at the recommendation.

The Cochrane Review

Many consider the methodology used in Cochrane Reviews to be the gold standard for systematic reviews. But what is a Cochrane Review and what is its relationship to the Cochrane Collaboration, the Cochrane Library, and the Cochrane Database of Systematic Reviews? 

The Cochrane Collaboration is an international voluntary organization that prepares, maintains and promotes the accessibility of systematic reviews of the effects of healthcare. Collaborative Review Groups, each focused on a particular topic area, produce and maintain systematic reviews. 

The Cochrane Library is a database from the Cochrane Collaboration that allows simultaneous searching of six EBM databases. The Cochrane Library is licensed by the Levy Library for the Mount Sinai community.

The Cochrane Database of Systematic Reviews (CDSR) is one of the six databases in the Cochrane Library. It contains the full-text of Cochrane Reviews and review protocols.  

Cochrane Reviews are systematic reviews authored by members of the Cochrane Collaboration and published in the Cochrane Database of Systematic Reviews. They address specific questions about the effects of clinical interventions: treatments, prevention, screening and rehabilitation. Meta-analysis may be performed.

Abstracts of completed Cochrane Reviews are freely available through PubMed and other open-access databases. The full-text of Cochrane Reviews is available to the Mount Sinai community from Cochrane Library. They are also available through databases such as PubMed via Levy Library's journal article linking service, FIND IT.