| Level of evidence | Study design |
|---|---|
| 1a | Systematic review of randomized controlled trial |
| 1b | Randomized controlled trial |
| 1c | Case series |
| 2a | Systematic review of cohort studies |
| 2b | Individual cohort study (retrospective) |
| 2c | Outcomes research/ecological studies |
| 3a | Systematic review of case-control studies |
| 3b | Individual case cohort study |
| 4 | Case series and case-control studies |
| 5 | Expert opinion |