Hierarchy of evidence

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Evidence hierarchies reflect the relative authority of various types of biomedical research. Although there is no single, universally-accepted hierarchy of evidence,[1] there is broad agreement on the relative strength of the principal types of research, or epidemiological studies. Randomized controlled trials (RCTs) rank above observational studies, while expert opinion and anecdotal experience are ranked at the bottom. Some evidence hierarchies place systematic review and meta analysis above RCTs, since these often combine data from multiple RCTs, and possibly from other study types as well. Evidence hierarchies are integral to evidence-based medicine.

The use of evidence hierarchies has been criticized as allowing RCTs too much authority. Not all research questions can be answered through RCTs, either because of practical issues or because of ethical issues. Moreover, even when evidence is available from high-quality RCTs, evidence from other study types may still be relevant.

Greenhalgh suggests[2][3] that

"the relative weight carried by the different types of primary study when making decisions about clinical interventions (the “hierarchy of evidence”) puts them in the following order:

  1. Systematic reviews and meta-analyses of "RCTs with definitive results".
  2. RCTs with definitive results (confidence intervals that do not overlap the threshold clinically significant effect)
  3. RCTs with non-definitive results (a point estimate that suggests a clinically significant effect but with confidence intervals overlapping the threshold for this effect)
  4. Cohort studies
  5. Case-control studies
  6. Cross sectional surveys
  7. Case reports


  1. ^ Two examples of evidence hierarchies can be found in the qualification of evidence section of the Wikipedia article on evidence-based medicine.
  2. ^ Greenhalgh, T (July 1997). "How to read a paper. Getting your bearings (deciding what the paper is about)". BMJ. 315 (7102): 243–6. doi:10.1136/bmj.315.7102.243. PMC 2127173Freely accessible. PMID 9253275. 
  3. ^ Guyatt, GH; Sackett, DL; Sinclair, JC; Hayward, R; Cook, DJ; Cook, RJ (December 1995). "Users' guides to the medical literature. IX. A method for grading health care recommendations. Evidence-Based Medicine Working Group". JAMA. 274 (22): 1800–4. doi:10.1001/jama.1995.03530220066035. PMID 7500513. 

Further reading[edit]

External links[edit]

  • The American Society for Experimental NeuroTherapeutics, Inc, "Observational Versus Experimental Studies: What’s the Evidence for a Hierarchy?"