Table 1.

Levels of evidence

Level of evidence	Type of study
1a	Systematic review of randomized controlled trials
1b	Individual randomized controlled trial (with narrow confidence interval)
1c	All cases affected before intervention, some or none affected after intervention
2a	Systematic review of cohort studies
2b	Individual cohort study (including low-quality randomized controlled trial)
2c	‘Outcomes’ research
3a	Systematic review 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’

Adapted with permission from reference 1