Table 2.

Levels of evidence: research design rating

Source: Canadian Task Force on Periodic Health Examination. The Canadian guide to clinical preventive health care. Ottawa, Canada: Health Canada, 1994.
I Evidence from randomized controlled trial(s)
II-1 Evidence from controlled trial(s) without randomization
II-2 Evidence from cohort or case-control analytic studies, preferably from more than one center or research group
II-3 Evidence from comparisons between times or places with or without the intervention; dramatic results in uncontrolled experiments could be included here
III Opinions of respected authorities, based on clinical experience; descriptive studies or reports of expert committees