| 1A. Systematic review of randomized clinical trials (RCTs) (with consistent results from individual studies) |
| 1B. RCTs (of good quality) |
| 2A. Systematic review of 2B studies (with consistent results from individual studies) |
| 2B. Prospective and comparative studies (or RCTs of poorer quality) |
| 2C. Outcome studies (e.g., analyses of large registries, population-based data) |
| 3. Retrospective and comparative studies, case–control studies |
| 4. Case series (i.e., studies without a control group) |
| 5. Expert opinion, animal or lab experiments. |
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