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Levels of evidence for therapeutic studies adapted from material published by the Centre for Evidence Based Medicine, Oxford, UK (www.cebm.net)
| Level I | Randomised controlled trials + systematic reviews level I studies |
| Level II | Prospective cohort studies + systematic reviews level II studies |
| Level III | Retrospective cohort studies + systematic reviews level III studies |
| Level IV | Case series |
| Level V | Expert opinion |
