A | At least one high quality meta-analysis, systematic review of RCTs, or RCT with a very low risk of bias, and directly applicable to the target population; or a well conducted meta analysis, systematic review of RCTs, or RCT with a low risk of bias or a body of evidence consisting principally of such studies, directly applicable to the target population, and demonstrating overall consistency of results |
B | A body of evidence including 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, directly applicable to the target population, and demonstrating overall consistency of results; or extrapolated evidence from high quality systematic reviews of case–control or cohort studies, or from 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 |
C | A body of evidence including 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, directly applicable to the target population and demonstrating overall consistency of results; or extrapolated evidence from high quality systematic reviews of case–control or cohort studies, or from 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 |
D | Non-analytic studies—for example, case reports, case series, or expert opinion; or extrapolated evidence from 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 |
√ | No evidence exists; recommended best practice based on the clinical experience of the guideline development group. |