Table 3.

Levels of evidence

1a	Systematic Review of Randomized Control Trials
1b	Individual randomized control trials (with narrow confidence interval)
1c	All or none case-series
2a	Systematic review of cohort studies
2b	Individual cohort study (including low quality RCT; eg, <80% follow-up)
2c	Outcomes research or ecological studies
3a	Systematic review 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”