Table 1.
The hierarchy of evidence from the center for evidence-based-medicine
| Level | Definition |
|---|---|
| 1a | Systematic review (with homogeneity) of randomized clinical trials (RCTs) |
| 1b | Individual RCT (with narrow confidence interval) |
| 2a | Systematic review (with homogeneity) of cohort studies |
| 2b | Individual cohort study |
| 3a | Systematic review (with homogeneity) of case–control studies |
| 3b | Individual case–control study |
| 4 | Case-series (and poor quality cohort and case–control studies) |
| 5 | Expert opinion without explicit critical appraisal, or based on physiology, bench research or “first principles” |