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 |
3 | Non-analytical studies, e.g. case reports, case series |
4 | Expert opinion |