Levels of Evidence
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.
Levels of Evidence
Grades of Recommendation
| 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 |
Strength of Recommendation Taxonomy (SORT)
- SORTThe 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.