Table 1.

Sackett’s classification of level of evidence

Level	Type of trial	Criteria for classification
I	Large, randomized trials with clear-cut results	Sample size calculation, study end point
II	Small, randomized trials	Matched analysis
III	Nonrandomized, contemporaneous controls	Noncomparative, prospective
IV	Nonrandomized, historical controls	Retrospective analysis, cohort studies
V	No control, case series only, opinion of experts	Small series, review articles