Levels of Evidence


Evidence obtained from at least one properly randomized controlled trial.


Evidence obtained from well-designed controlled trials without randomization.


Evidence obtained from well-designed cohort or case-control analytic studies, preferably from more than one center or research group.


Evidence obtained from multiple time series with or without the intervention. Dramatic results in uncontrolled experiments (such as the results of the introduction of penicillin in the 1940s) could also be regarded as this type of evidence.


Opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees.

* Canadian Task Force on the Periodic Health Examination: The periodic health examination: 2. 1987 update. Can Med Assoc J 1988;138:618-26.