Table 2

Evidence levels according to the Scottish Intercollegiate Guidelines Network

1Meta-analyses and systematic reviews of randomised clinical trials
1++Very low risk of bias
1+Low risk of bias
1−High risk of bias
2Systematic reviews of cohort or case and control studies
2++Very low risk of bias and high probability of a causal relationship
2+Low risk of bias and moderate probability of a causal relationship
2−High risk of bias and significant risk that the relationship is not causal
3Non-analytical studies, such as case reports and case series
4Expert opinion