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. 2016 Jul 8;16:168. doi: 10.1186/s12909-016-0700-7

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