Table 1. Classification of evidence system based on the Oxford Centre for Evidence-Based Medicine (OCEBM) Levels of Evidence.

1A	Systematic review (with homogeneity) of RCTs
1B	Individual RCT (with narrow confidence intervals)
2A	Systematic review (with homogeneity) of cohort studies
1C	All or none study
2B	Individual cohort study including low quality RCT
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 study)
5	Expert opinion without explicit critical appraisal or based on physiology bench research or “first principles”