Table 2

Scottish Intercollegiate Guidelines Network: key to evidence statements55

LevelCriteria
1++High quality meta-analyses, systematic reviews of randomised controlled trials (RCTs) or RCTs with a very low risk of bias
1+Well-conducted meta-analyses, systematic reviews or RCTs with a low risk of bias
1−Meta-analyses, systematic reviews or RCTs with a high risk of bias
2++High quality systematic reviews of case–control or cohort studies
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
2+Well-conducted case–control or cohort studies with a low risk of confounding or bias and a moderate probability that the relationship is causal
2−Case–control or cohort studies with a high risk of confounding or bias and a significant risk that the relationship is not causal
3Non-analytical studies, eg, case reports, case series
4Expert opinion