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Center of evidence-based medicine: levels of evidence.
| Level | Definition |
|---|---|
| 1a | Systematic reviews of RCTs |
| 1b | Individual RCT |
| 1c | All-or-none studies |
| 2a | Systematic reviews of cohort studies |
| 2b | Individual cohort studies or low-quality RCTs |
| 2c | Outcomes research |
| 3a | Systematic reviews of case–control studies |
| 3b | Individual case–control studies |
| 4 | Case series, poorly designed cohort, or case–control studies |
| 5 | Animal and bench research, expert opinion |
