Table 1.

Levels of evidence for therapeutic studies

Level	Type of evidence
1A	Systematic reviews (with homogeneity) of randomised controlled trials
1B	Individual randomised controlled trials (with narrow confidence intervals)
1C	All or none randomised controlled trials
2A	Systematic review (with homogeneity) of cohort studies
2B	Individual cohort study or low-quality randomised controlled trials (e.g. <80% follow-up)
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 study)
5	Expert opinion without explicit critical appraisal, or based on physiology, bench research or ‘first principles’