Table 1.
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 |