Hierarchy of Evidence

Not all evidence is judged to be of equal value, that is, there are hierarchies of research design that are evaluated to have different strengths, different levels of value in the decision making process.  See the two charts below – one graphically represented, one textually to help understand the concepts important to critical appraisal, assessment, and evaluation of research.

The Evidence Pyramid

 

[Source: Source: SUNY Downstate Medical Center. Medical Research Library of Brooklyn. Evidence Based Medicine Course. A Guide to Research Methods: The Evidence Pyramid: http://servers.medlib.hscbklyn.edu/ebm/2100.htm]

For a definition of the different types of research methodology, i.e. systematic review, randomized controlled trial, and others, visit the Medical Research Library of Brooklyn's Evidence Based Medicine tutorial.

Levels of Evidence

• Category I:  Evidence from at least one properly randomized controlled trial.
• Category II-1: Evidence from well-designed controlled trials without randomization.
• Category II-2: Evidence from well-designed cohort or case-control analytic studies, preferably from more than one center or research group.
• Category II-3: Evidence from multiple times series with or without intervention or dramatic results in uncontrolled experiments such as the results of the introduction of penicillin treatment in the 1940s.
• Category III: Opinions of respected authorities, based on clinical experience, descriptive studies and case reports, or reports of expert committees.

[Source: Harris, R.P. et al. (2001). Current methods of the U.S. Preventive Services Task Force: a review of the process. American Journal of Preventive Medicine. April 20 (3 Supplement): 21-35.]