Revised SIGN grading system: levels of evidence
| 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 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 relationship 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 relationship is causal. |
| 2− | Case-control or cohort studies with a high risk of confounding, bias, or chance and a significant risk that the relationship is not causal. |
| 3 | Non-analytical studies (e.g. case reports, case series). |
| 4 | Expert opinion |