Table 2 Adapted Oxford Centre for Evidence-Based Medicine levels of evidence
Level | Design | Tier |
|---|---|---|
1 | Systematic review of randomized controlled trials or individual randomized controlled trial | high |
2 | Systematic review of cohort studies or individual cohort study | high |
3 | Systematic review of case-control studies or individual case-control study | high |
4 | Case-series | low |
5 | Expert opinion | low |
6 | Unclear method | low |