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Levels of Evidence
Critically-appraised individual articles and synopses include:
- 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
- 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.
Resources That Rate The Evidence
Grades of Recommendation
|Type of Study
||Systematic review of (homogeneous) randomized
||Individual randomized controlled trials (with narrow
||Systematic review of (homogeneous) cohort studies
of "exposed" and "unexposed" subjects
||Individual cohort study / low-quality randomized
||Systematic review of (homogeneous) case-control studies
||Individual case-control studies
||Case series, low-quality cohort or case-control studies
||Expert opinions based on non-systematic reviews of
results or mechanistic studies
Strength of Recommendation Taxonomy (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.