Levels of evidence by study design for therapeutic trials and preventive programmes
| Level | Type of evidence |
| 1A | Systematic review (with homogeneity) of RCTs |
| 1B | Individual RCT (with narrow CIs) |
| 1C | All or none study |
| 2A | Systematic review (with homogeneity) of cohort studies |
| 2B | Individual cohort study (including low-quality RCT, eg, <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’ |
From the Centre for Evidence-Based Medicine (https://www.cebm.net/).
RCT, randomised controlled trial.