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Oxford levels of evidence
| Level | Study design |
|---|---|
| 1A | Systematic review of randomized controlled trials |
| 1B | Individual randomized controlled trial |
| 1C | All or none case‐series |
| 2A | Systematic review of cohort studies |
| 2B | Individual cohort study |
| 2C | Outcomes research |
| 3A | Systematic review of case–control studies |
| 3B | Individual case–control study |
| 4 | Case‐series |
| 5 | Expert opinion |
