| Level | Type of evidence |
|---|---|
| 1a | Systematic review of randomised controlled trials |
| 1b | Individual randomised controlled trials |
| 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 without critical appraisal |
| Adapted from Phillips et al., 1998. | |
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