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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.

Systematic Reviews

A systematic review is "a review that uses explicit, systematic methods to collate and synthesise findings of studies that address a clearly formulated question".1

Systematic reviews are useful for:

  • confirming current practices
  • guiding decision-making
  • informing future research

Meta-analyses, while often part of systematic reviews, are not interchangeable with them. They use statistical analysis to combine data from the studies found in the systematic review process. These studies must be homogenous enough that the data from them can be pooled together.

Limitations of systematic reviews include:

  • That since they are focused on a clearly formulated question, their conclusions only answer that particular question and cannot be generalized
  • That the synthesis of materials is only as reliable as the primary studies the review analyzed


  1. Page MJ, McKenzie JE, Bossuyt PM, Boutron I, Hoffmann TC, Mulrow CD, Shamseer L, Tetzlaff JM, Akl EA, Brennan SE, Chou R. The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. BMJ. 2021 Mar 29;372.
  2. Garg AX, Hackam D, Tonelli M. Systematic review and meta-analysis: when one study is just not enough. Clinical Journal of the American Society of Nephrology. 2008 Jan 1;3(1):253-60.

Primary Studies

In the absence of a current, well designed systematic review is not available, practitioners turn to the primary studies to answer their questions. The best research design depends on the question type. The table below lists optimal study methodologies for common types of clinical questions.

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" diagnostic test

(Natural History)

  • Cohort study

Etiology or Harm

  • Randomized controlled trial (RCT)
  • If RCTs are impractical or unethical, cohort study or case control study


  1. Oxford Centre for Evidence-Based Medicine Working Group Oxford (OCEBM). The Oxford Levels of Evidence 2 [Internet]. University of Oxford, Oxford UK; [cited 2021 Dec 13]. Available from:
  2. Burns PB, Rohrich RJ, Chung KC. The levels of evidence and their role in evidence-based medicine. Plast Reconstr Surg. 2011 Jul;128(1):305-310. doi: 10.1097/PRS.0b013e318219c171. PMID: 21701348; PMCID: PMC3124652.
  3. ACP Journal Club Inclusion Criteria [Internet]. American College of Physicians (ACP); [updated 2013 Nov 14; cited 2021 Dec 13]. Available from: