Summary of “The Oxford Levels of Evidence 2.” (adapted from OCEBM 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 |