1A

Systematic review (with homogeneity) of RCTs

1B

Individual RCT (with narrow confidence intervals)

1C

All or none study

2A

Systematic review (with homogeneity) of cohort studies

2B

Individual cohort study (including low quality RCT, 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 study

5

Expert opinion without explicit critical appraisal or based on physiology bench research or “first principles”