Oxford Centre for Evidence-based Medicine 2011 Levels of Evidence for interventions33
| Level 1 | Systematic reviews |
| Level 2 | Randomised controlled trials with low/moderate risk of bias or observational studies with dramatic effect |
| Level 3 | Non-randomised controlled trials with low/moderate risk of bias or randomised controlled trials at high risk of bias |
| Level 4 | Case series, case-control studies, historically controlled studies or non-randomised controlled trials at high risk of bias |
| Level 5 | Mechanism-based reasoning/expert opinion |