TABLE I.
Levels of evidencea
| Level | Description |
|---|---|
| 1A | Systematic review with randomized controlled trials |
| 1B | Individual randomized controlled trial (with narrow confidence interval) |
| 1C | All-or-none case series |
| 2A | Systematic review with homogeneity of cohort studies |
| 2B | Individual cohort study |
| 2C | “Outcomes” research; ecological studies |
| 3A | Systematic review with homogeneity of case–control studies |
| 3B | Individual case–control study |
| 4 | Case series |
| 5 | Expert opinion without explicit critical appraisal, or based on physiology, bench research, or “first principles,” or descriptive epidemiology |
a
Adapted from the Centre for Evidence Based Medicine, Oxford Centre for Evidence-based Medicine, Levels of Evidence6.