Table 1 The evidence laddera
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