Table 1 Five levels of evidence for therapeutic studies (from the Centre for Evidence-Based Medicine, http://www.cebm.net)

1A: Systematic review (with homogeneity) of RCTs

1B: Individual RCT (with narrow confidence intervals)

1C: All or none study

2A: Systematic review (with homogeneity) of cohort studies

2B: Individual Cohort study (including low quality RCT, e.g., < 80% follow-up)

2C: “Outcomes” research; Ecological studies

3A: Systematic review (with homogeneity) of case–control studies

3B: Individual Case–control study

4: Case series and poor-quality cohort and case–control study

5: Expert opinion without explicit critical appraisal or based on physiology bench research or “first principles”