Table 1.

Sackett's classification of the level of evidence according to the modification of Heinrich et al[10,11]

Level of evidence	Type of trial	Criteria for classification	Grade of recommendation
I	Large randomized trials with clear-cut results (and low error risk)	Sample size calculation provided and fulfilled; study endpoint provided	A
II	Small randomized trials with uncertain results (and moderate to high error risk)	Matched analysis, sample size calculation not given or not fulfilled; study endpoints not provided; convincing comparative studies	B
III	Nonrandomized, contemporaneous controls	Noncomparative, prospective	C
IV	Nonrandomized, historical controls	Retrospective analysis, cohort studies	-
V	V No control, case series only; experts opinions	Small series, review articles	-