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Clerkship Resources: Hierarchy of Evidence & Optimal Study Types

Keys to success in the clerkships and beyond.

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 Pyramid

The Evidence Pyramid. Image courtesy of: SUNY Downstate Medical Center. Medical Research Library of Brooklyn. Evidence Based Medicine Course. A Guide to Research Methods: The Evidence Pyramid. Available from:

ACP's Selection Criteria for Articles

The American College of Physicians has established criteria for selecting studies for review in the ACP Journal Club. Their standards reviewing articles on common clinical questions are listed below; the original is available here (1). 

The ACP criteria for clinical studies are useful for suggesting search terms in an unfiltered database such as Google Scholar, and for reviewing search results from any database to select articles likely to to provide the best evidence.

Question Type

ACP's Criteria for Primary Studies

Treatment & Prevention

  • Random allocation of participants to comparison groups
  • Follow-up (end-point assessment) of at least 80% of those entering the investigation
  • Outcome measure of known or probable clinical importance


  • Inclusion of a spectrum of participants, some but not all of whom have the disorder or derangement of interest
  • Objective diagnostic ("gold") standard OR current clinical standard for diagnosis, preferably with documentation of reproducible criteria for subjectively interpreted diagnostic standard (i.e., report of statistically significant measure of agreement beyond chance among observers)
  • Each participant must receive both the new test and some form of the diagnostic standard
  • Interpretation of diagnostic standard without knowledge of test result
  • Interpretation of test without knowledge of diagnostic standard result

(Natural History)

  • Inception cohort of individuals, all initially free of the outcome of interest
  • Follow-up of at least 80% of patients until the occurrence of a major study end point or to the end of the study

Etiology or Harm

  • Exploration of the relationship between exposures and putative clinical outcomes
  • Prospective data collection with clearly identified comparison groups for those at risk of the outcome of interest (in descending order of preference from randomized controlled trial, quasi-randomized controlled trial, nonrandomized controlled trial, cohort studies with case-by-case matching or statistical adjustment to create comparable groups, to nested case-control studies)
  • Masking of observers of outcomes to exposures (criterion assumed to be met if outcome is objective, i.e., all-cause mortality, objective test)