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Levels of Evidence
| Level 1a = | Systematic Review of Randomized |
| Controlled Trials (RCTs) | |
| Level 1b = | RCTs with Narrow Confidence Interval |
| Level 1c = | All or None Case Series |
| Level 2a = | Systematic Review Cohort Studies |
| Level 2b = | Cohort Study/Low Quality RCT |
| Level 2c = | Outcomes Research |
| Level 3a = | Systematic Review of Case-Controlled Studies |
| Level 3b = | Case-Controlled Study |
| Level 4 = | Case Series, Poor Cohort Case Controlled |
| Level 5 = | Expert Opinion |
