Table 2.

Oxford Centre for Evidence-Based Medicine—levels of evidence

Level
1a	Systematic review (with homogeneity) of RCTs
1b	Individual RCT (with narrow CI)
1c	‘All or none’
2a	Systematic review (with homogeneity) of cohort studies
2b	Individual cohort study (including low-quality RCT)
2c	‘Outcomes’ research, ecological studies
3a	Systematic review (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 of ‘first principles’

RCT, randomised controlled trial.