Table 2.

Levels of evidence according to the Oxford Centre for evidence-based medicine

Level of evidence	Type of study
1a	SR of RCTs
1b	Individual RCT (with narrow confidence intervals)
1c	Clinical practice (all or none)
2a	SR of cohort studies
2b	Individual cohort study (including low quality RCTs; e.g., <80% follow-up)
2c	Outcomes research; ecological studies
3a	SR (with homogeneity*) 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”

SR = systematic review; RCT = randomized clinical trial.