Levels of Evidence for Treatment Efficacy
| 1a | Systematic review of random controlled trials (RCTs) |
| 1b | Individual RCT with narrow confidence interval |
| 1c | All or none |
| 2a | Systematic review of cohort studies |
| 2b | Individual cohort study (including low quality RCT; e.g., <80% follow-up)* |
| 2c | “Outcomes” research; ecological studies |
| 3a | Systematic review of case-control studies |
| 3b | Individual case-control study |
| 4 | Case-series (and poor-quality cohort and case-control studies†) |
| 5 | Expert opinion without explicit critical appraisal, or based on physiology, bench research, or first principles |
Adapted from the Oxford Centre for Evidence-Based Medicine.32
* Quasi-experimental design (i.e. non-equivalent comparison group studies) match cohort studies and therefore meet level 2b evidence.33
† Poor-quality cohort study refers to cohort studies that did not clearly define comparison groups and/or did not measure exposures and outcomes in the same way in both exposed and nonexposed individuals, did not identify or appropriately control known confounders, and failed to carry out a sufficiently long and complete follow-up of patients.