Levels of Evidence | Type of Evidence |
---|---|
Ia | Systematic review (with homogeneity)b of level 1 studiesc |
Ib | Level 1 studiesc |
II | Level 2 studiesd |
Systematic reviews of level 2 studies | |
III | Level 3 studiese |
Systematic reviews of level 3 studies | |
IV | Consensus, expert committee reports or opinions, and/or clinical experience without explicit critical appraisal; or based on physiology, bench research, or “first principles” |
↵a Adapted from Reference 3a.
↵b “Homogeneity” means there are no or minor variations in the directions and degrees of results between individual studies that are included in the systematic review.
↵c Level 1 studies are studies:
that use a blind comparison of the test with a validated reference standard
in a sample of patients that reflects the population to whom the test would apply.
↵d Level 2 studies are studies that have only 1 of the following:
narrow population (the sample does not reflect the population to whom the test would apply).
use a poor reference standard (defined as that where the “test” is included in the “reference,” or where the “testing” affects the “reference”).
the comparison between the test and reference standard is not blind.
case–control studies.
↵e Level 3 studies are studies that have at least 2 or 3 of the features listed above.