| High-quality systematic reviews |
| Large randomized trials with clear-cut results |
| Small randomized trials with uncertain results (i.e., positive trends without statistical significance) |
| Nonrandomized trials with contemporary controls |
| Nonrandomized trials with historical controls |
| Cohort studies |
| Case-control studies |
| Dramatic results from uncontrolled studies (e.g., the treatment of infections with penicillin in the 1940s) |
| Case series and other descriptive studies |
| Reports of expert committees and opinions of respected authorities, based on clinical experience |
aBased on references 3, 4 and 5