Table 1 Classification system for levels of evidence and grades of recommendations

I

Evidence obtained from a meta-analysis of multiple, well-designed, controlled studies. Randomized trials with low false-positive and low false-negative errors (high power)

II

Evidence obtained from at least 1 well-designed experimental study. Randomized trials with high false-positive and/or false-negative errors (low power)

III

Evidence obtained from well-designed, quasi-experimental studies such as non-randomized, controlled single-group, pre–post, cohort, time series, or matched case-control series

IV

Evidence from well-designed, non-experimental studies, such as comparative and correlational descriptive and case studies

V

Evidence from case reports and clinical samples

Grade

Type of evidence

A

Evidence of type I or consistent findings from multiple studies of type II, III, or IV

B

Evidence of type II, III, or IV, and findings are generally consistent

C

Evidence of type II, III, or IV, but findings are inconsistent

D

Minimal or no systematic empirical evidence