Table 1 Categories of evidence in studies

1a

Systematic review and RCT meta-analysis

1b

At least one RCT with narrow confidence interval

2a

Systematic review and meta-analysis of cohort studies

2b

At least one cohort study or low quality RCT

3a

Systematic review and meta-analysis of case-control studies

3b

At least one case control study

4

At least one case series or cohort study and low quality case-control studies

5

Expert opinion without critical evaluation explicit or based on physiology, bench research or “fundamental principles”