Research & Subject Guides: Evidence-Based Medicine: Levels of Evidence

Resources That Rate The Evidence

Grade of Recommendation	Level of Evidence	Type of Study
A	1a	Systematic review of (homogeneous) randomized controlled trials
A	1b	Individual randomized controlled trials (with narrow confidence intervals)
B	2a	Systematic review of (homogeneous) cohort studies of "exposed" and "unexposed" subjects
B	2b	Individual cohort study / low-quality randomized control studies
B	3a	Systematic review of (homogeneous) case-control studies
B	3b	Individual case-control studies
C	4	Case series, low-quality cohort or case-control studies
D	5	Expert opinions based on non-systematic reviews of results or mechanistic studies

Critically-appraised individual articles and synopses include:

Filtered evidence:

Level I: Evidence from a systematic review of all relevant randomized controlled trials.
Level II: Evidence from a meta-analysis of all relevant randomized controlled trials.
Level III: Evidence from evidence summaries developed from systematic reviews
Level IV: Evidence from guidelines developed from systematic reviews
Level V: Evidence from meta-syntheses of a group of descriptive or qualitative studies
Level VI: Evidence from evidence summaries of individual studies
Level VII: Evidence from one properly designed randomized controlled trial

Unfiltered evidence:

Level VIII: Evidence from nonrandomized controlled clinical trials, nonrandomized clinical trials, cohort studies, case series, case reports, and individual qualitative studies.
Level IX: Evidence from opinion of authorities and/or reports of expert committee

Two things to remember:

1. Studies in which randomization occurs represent a higher level of evidence than those in which subject selection is not random.

2. Controlled studies carry a higher level of evidence than those in which control groups are not used.

SORT
The American Academy of Family Physicians uses the Strength of Recommendation Taxonomy (SORT) to label key recommendations in clinical review articles. In general, only key recommendations are given a Strength-of-Recommendation grade. Grades are assigned on the basis of the quality and consistency of available evidence.