Classification of levels of evidence
| LEVEL | TYPES OF EVIDENCE |
|---|---|
| I | High-quality RCT with statistically significant difference or no statistically significant difference but narrow CIs |
| Systematic review of level I RCTs (and study results were homogenous) | |
| II | Lower-quality RCT (eg, < 80% follow-up, no blinding, or improper randomization) |
| Prospective comparative study | |
| Individual cohort study | |
| Systematic review of level II studies or level I studies with inconsistent results | |
| III | Case-control study |
| Retrospective comparative study | |
| Systematic review of level III studies | |
| IV | Case series |
| V | Expert opinion |
CI—confidence interval, RCT—randomized controlled trial.
Adapted from the Oxford University Centre for Evidence-Based Medicine.7