Table 1.

Hierarchies of evidence for questions of therapy, prevention, aetiology or harm [CEBM]

Level 1a:	Systematic reviews (with homogeneity) of randomized controlled trials (RCTs)
Level 1b:	Individual RCTs (with narrow confidence interval)
Level 1c:	All or none studies
Level 2a:	Systematic reviews (with homogeneity) of cohort studies
Level 2b:	Individual cohort study or low quality RCTs (e.g. <80 % follow-up)
Level 2c:	“Outcomes” Research; ecological studies
Level 3a:	Systematic review (with homogeneity) of case-control studies
Level 3b:	Individual case-control study
Level 4:	Case-series (and poor quality cohort and case-control studies)
Level 5:	Expert opinion without explicit critical appraisal, or based on physiology, bench research or ‘first principles’