Level		Description
1	a	Systematic reviews (with homogeneity) of randomized controlled trials
	b	Individual randomized controlled trials (with narrow confidence intervals)
	c	All or none randomized controlled trials
2	a	Systematic reviews (with homogeneity) of cohort studies
	b	Individual cohort study or low quality randomized controlled trials (e.g. <80% follow-up)
	c	“Outcomes” research; ecological studies
3	a	Systematic review (with homogeneity) of case-control studies
	b	Individual case-control study
	c	Cross-sectional studies
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”