Table 1 Score criteria for SIGN levels of evidence and grades of recommendation

1 +  + 

High-quality meta-analysis, systematic reviews of RCTs or RCTs with very low risk of bias

A

At least one meta-analysis, systematic review or RCT rated as 1 +  + and directly applicable to the target population OR

1 + 

Well-conducted meta-analyses, systematic reviews of RCTs or RCTs with low risk of bias

A systematic review of RCTs or a body of evidence consisting principally of studies rated as 1 + directly applicable to the target population and demonstrating overall consistency of results

1-

Meta-analyses, systematic reviews or RCTs, or RCTs with high risk of bias

B

A body of evidence including studies rated as 2 +  + directly applicable to the target population and demonstrating overall consistency of results OR

2 +  + 

High-quality systematic reviews of case–control or cohort studies or

High-quality case–control or cohort studies with a very low risk of confounding, bias or chance and a high probability that the relationship is causal

Extrapolated evidence from studies rated as 1 +  + or 1 + 

2 + 

Well-conducted case–control or cohort studies with a low risk of confounding, bias or chance and a moderate probability that the relationship is causal

C

A body of evidence including studies rated as 2 + directly applicable to the target population and demonstrating overall consistency of results OR

2-

Case–control or cohort studies with a high risk of confounding, bias or chance and a significant risk that the relationship is not causal

Extrapolated evidence from studies rated as 2 +  + 

3

Non-analytic studies, e.g., case reports, case series

D

Evidence level 3 or 4 OR

4

Expert opinion

Extrapolated evidence from studies rated as 2 +