Table 1

 Revised SIGN grading system for levels of evidence

Level of evidence
1++High quality meta-analyses, systematic reviews of RCTs, or RCTs with a very low risk of bias
1+Well conducted meta-analyses, systematic reviews of RCTs, or RCTs with a low risk of bias
1−Meta-analyses, systematic reviews of RCTs, 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, bias, or chance, and a high probability that the relation is causal
2+Well conducted case-control or cohort studies with a low risk of confounding, bias, or chance and a moderate probability that the relation is causal
2−Case-control or cohort studies with a high risk of confounding, bias, or chance and a significant risk that the relation is not causal
3Non-analytical studies, e.g. case reports, case series
4Expert opinion