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University Library DePaul Library

Evidence Pyramid

Depending on their purpose, design, and mode of reporting or dissemination, health-related research studies can be ranked according to the strength of evidence they provide, with the sources of strongest evidence at the top, and the weakest at the bottom:


pyramid of levels of evidence

Secondary Sources: studies of studies

Systematic Review

  • Identifies, appraises, and synthesizes all empirical evidence that meets pre-specified eligibility criteria
  • Methods section outlines a detailed search strategy used to identify and appraise articles
  • May include a meta-analysis, but not required (see Meta-Analysis below)


  • A subset of systematic reviews: uses quantitative methods to combine the results of independent studies and synthesize the summaries and conclusions
  • Methods section outlines a detailed search strategy used to identify and appraise articles; often surveys clinical trials
  • Can be conducted independently, or as a part of a systematic review
  • All meta-analyses are systematic reviews, but not all systematic reviews are meta-analyses

Evidence-Based Guideline

  • Provides a brief summary of evidence for a general clinical question or condition
  • Produced by professional health care organizations, practices, and agencies that systematically gather, appraise, and combine the evidence
  • Click on the 'Evidence-Based Care Sheets' link located at the top of the CINAHL screen to find short overviews of evidence-based care recommendations covering 140 or more health care topics.

screenshot of CINAHL database


Meta-Synthesis or Qualitative Synthesis (Systematic Review of Qualitative or Descriptive Studies)

  • a systematic review of qualitative or descriptive studies, low strength level

Primary Sources: original studies

Randomized Controlled Trial

  • Experiment where individuals are randomly assigned to an experimental or control group to test the value or efficiency of a treatment or intervention

Non-Randomized Controlled Clinical Trial (Quasi-Experimental)

  • Involves one or more test treatments, at least one control treatment, specified outcome measures for evaluating the studied intervention, and a bias-free method for assigning patients to the test treatment

Case-Control or Case-Comparison Study (Non-Experimental)

  • Individuals with a particular condition or disease (the cases) are selected for comparison with individuals who do not have the condition or disease (the controls)

Cohort Study (Non-Experimental)

  • Identifies subsets (cohorts) of a defined population
  • Cohorts may or may not be exposed to factors that researchers hypothesize will influence the probability that participants will have a particular disease or other outcome
  • Researchers follow cohorts in an attempt to determine distinguishing subgroup characteristics

Further Reading