TABLE I.

Levels of evidence and grades of recommendations (adapted from Cook et al., 1992⁷)

Levels of evidence		Grades of recommendation
Level	Type of evidence	Grade	Recommendation
i	Evidence demonstrated by means of meta-analyses of well-designed controlled trials or large randomized trials with clear-cut results (low false-positive and false-negative errors, high power)	A	Supported by level i evidence or multiple level ii, iii, or iv trials presenting concordant observations
ii	Evidence demonstrated by means of small randomized trials with uncertain results (high false-positive and false-negative errors, low power)	B	Supported by level ii, iii, or iv trials presenting generally concordant observations
iii	Evidence demonstrated by means of nonrandomized concur- rent cohort comparisons with contemporaneous controls	C	Supported by level ii, iii, or iv trials presenting non-concordant observations
iv	Evidence demonstrated by means of nonrandomized historical cohort comparisons	D	Supported by little or no empiric evidence
v	Evidence demonstrated by means of case series without controls