Table 2.

Levels of Evidence (Oxford Centre for Evidence-based Medicine—March 2009) [107].

Level	Therapy/Prevention, Aetiology/Harm
1a	Systematic Review (with homogeneity) of Randomized Controlled Trials
1b	Individual Randomized Controlled Trial (with narrow Confidence Interval)
1c	All or none
2a	Systematic Review (with homogeneity) of cohort studies
2b	Individual cohort study (including low quality Randomized Controlled Trial; e.g., <80% follow-up)
2c	“Outcomes” Research; Ecological studies
3a	Systematic Review (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”