Hierarchy of Evidence for Studies of Therapy or Harm
| Quality | Type of Evidence |
|---|---|
| 1a (best) | Systematic review of RCTs |
| 1b | Individual RCT with narrow confidence interval |
| 1c | “All-or-none” case series |
| 2a | Systematic review of cohort studies |
| 2b | Individual cohort study |
| RCT with <80% follow-up | |
| 2c | Outcomes research |
| Ecological study | |
| 3a | Systematic review of case-control studies |
| 3b | Individual case-control study |
| 4 | Case series |
| 5 (worst) | Expert opinion |
RCT, randomized control trial.
Reprinted with permission from Gray GE: Evidence-Based Psychiatry. Arlington, VA: American Psychiatric Publishing, 2004