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Levels of evidence (Oxford Centre for Evidence-based Medicine, March 2009, www.cebm.net)
| 1a | Systematic review of RCT |
| 1b | Individual RCT |
| 1c | (All or none) |
| 2a | Systematic review of cohort studies |
| 2b | Individual cohort studies and poor-quality RCT |
| 2c | ("Outcomes" research; ecological studies) |
| 3a | Systematic review of case-control studies |
| 3b | Individual case-control studies |
| 4 | Case series (and poor-quality case-control or cohort studies) |
RCT, randomized controlled study
