Summary of ‘The Oxford Levels of Evidence 2’. (Adapted from Oxford Center for Evidence-Based Medicine Levels of Evidence Working Group)
| Level 1 | Level 2 | Level 3 | Level 4 | Level 5 |
| Systematic review or meta-analysis | Randomized trial or observational study with dramatic effect | Non-randomized, controlled cohort, or follow-up study | Case series, case-control, or historically controlled study | Mechanism-based reasoning |