Revised SIGN grading system: levels of evidence
| I++ | High quality meta-analyses, systematic reviews of randomised controlled trials (RCTs), or RCTs with a very low risk of bias |
| I+ | Well conducted meta-analyses, systematic reviews of RCTs, or RCTs with a low risk of bias |
| I− | Meta-analyses, systematic reviews of RCTs, or RCTs with a high risk of bias |
| II++ | 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 relationship is causal |
| II+ | Well conducted case-control or cohort studies with a low risk of confounding, bias, or chance and a moderate probability that the relationship is causal |
| II− | Case-control or cohort studies with a high risk of confounding, bias, or chance and a significant risk that the relationship is not causal |
| III | Non-analytical studies, e.g. case reports, case series |
| IV | Expert opinion |