Table 17– Rating level of evidence according to the Scottish Intercollegiate Guidelines Network (SIGN)
RatingStudy designSpecial conditionsLevel of evidence
1++High quality meta-analyses, systematic reviews of RCTs, or RCTs with a very low risk of biasIf directly applicable to target populationA
1+Well-conducted meta-analyses, systematic reviews, or RCTs with a low risk of biasIf directly applicable to target population and overall consistency of results
1++ or 1+Extrapolated evidenceB
2++High quality systematic reviews of case–control or cohort or studiesIf directly applicable to target population and overall consistency of results
High quality case–control or cohort studies with a very low risk of confounding or bias and a high probability that the relationship is causal
Extrapolated evidenceC
2+Well-conducted case–control or cohort studies with a low risk of confounding or bias and a moderate probability that the relationship is causalIf directly applicable to target population and overall consistency of results
Extrapolated evidenceD
3Nonanalytic studies, e.g. case reports, case series
4Expert opinion
1-Meta-analyses, systematic reviews, or RCTs with a high risk of biasNo supporting evidence
2-Case control or cohort studies with a high risk of confounding or bias and a significant risk that the relationship is not causal
  • RCT: randomised controlled trial.