SIGN levels 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 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 or 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 |
| 3 | Non-analytic studies, eg, case reports, case series |
| 4 | Expert opinion |
RCT, randomised controlled trial.