Grade | 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 or RCTs or RCTs with a high risk of bias |
2++ | High-quality systematic reviews of case–control or cohort studies or 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-analytic studies, for example, case reports, case series |
4 | Expert opinion |
RCT, randomised controlled trial.