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Evidence Synthesis : Levels of Evidence

Useful resources and tools for planning and conducting systematic reviews

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

Triangle with labels of levels of evidence top down it reads, MA or meta analysis, SR or systematic review, RCT or randomized controlled trials, Cohort Studies, Case control studies, Case series/ Case reports, expert opinion, animal research studies

Grades of Recommendation

Grade of
Level of
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)