Table 1.

Levels of evidence for studies of therapeutic interventions from the Oxford Centre for Evidence-based Medicine (website title http://www.cebm.net/levels_of_evidence.asp) [14]

Level		Study design
1	A	Systematic review (with homogeneity) of randomized controlled trials
	B	Individual randomized controlled trial (with narrow confidence interval)
	C	All or none
2	A	Systematic review (with homogeneity) of cohort studies
	B	Individual cohort study (including low quality randomized controlled trial, 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
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"