Table 1.

Levels of evidence and grades of recommendation

Level	Source of evidence	Grade	Grade of recommendation
I	Meta-analysis of multiple well-designed, controlled studies; randomized trials with low rates of false-positive and low false-negative errors (high power)	A	Evidence rated as level I or consistent findings from multiple studies at levels II, III, or IV
II	At least one well-designed experimental study; randomized trials with high rates of false-positive and high false-negative errors (low power)	B	Evidence at levels II, III or IV, and generally consistent findings
III	Well-designed, quasi-experimental studies such as non-randomized, controlled, single-group, preoperative and correlation descriptive studies, and case studies	C	Evidence at levels II, III or IV, but inconsistent findings
IV	Well-designed, non-experimental studies such as comparative and correlation descriptive studies, and case studies	D	Little or no systematic empirical evidence
V	Case reports and clinical examples