Table 1 Level of evidence (with permission from the SIGN, from section 6 of the SIGN50, 2004) [15, 16]

A

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

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

B

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

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

C

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

Extrapolated evidence from studies rated as 2⁺⁺

D

Evidence level 3 or 4; or

Extrapolated evidence from studies rated as 2⁺