Table 4.

Oxford levels of evidence [26]

Level	Therapy/prevention/etiology/harm
1a	Systematic review (SR) (with homogeneity*) of RCTs
1b	Individual randomized controlled trial (RCT) (with narrow confidence interval”)
1c	All or none§
2a	SR (with homogeneity*) of cohort studies
2b	Individual cohort study (including low quality RCT; eg, < 80% follow-up)
2c	“Outcomes” Research; Ecological studies
3a	SR (with homogeneity*) 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”

See Table 6 for definition of symbols