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Grades of clinical evidence
| Grade | Qualifying level of evidence |
|---|---|
| 1a | Meta-analysis of RCTs |
| 1b | Single RCT |
| 2a | Systematic review of cohort studies |
| 2b | Single cohort study Single RCT of limited value |
| 3a | Systematic review of case studies |
| 3b | Single case–control study |
| 4 | Case series Case cohort studies Single cohort study of limited value |
| 5 | Expert opinion without explicit critical appraisal, or based on physiology, bench research or first principles |
RCT, randomised controlled trial.
