Oxford Centre for Evidence-Based Medicine—levels of evidence
| Level | |
| 1a | Systematic review (with homogeneity) of RCTs |
| 1b | Individual RCT (with narrow CI) |
| 1c | ‘All or none’ |
| 2a | Systematic review (with homogeneity) of cohort studies |
| 2b | Individual cohort study (including low-quality RCT) |
| 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 studies) |
| 5 | Expert opinion without explicit critical appraisal, or based on physiology, bench research of ‘first principles’ |
RCT, randomised controlled trial.