1++ | High quality meta-analyses, systematic reviews of randomised controlled trials or randomised controlled trials with a very low risk of bias |
1+ | Well conducted meta-analyses, systematic reviews of randomised controlled trials or randomised controlled trials with a low risk of bias |
1– | Meta-analyses, systematic reviews or randomised controlled trials with a high risk of bias |
2++ | High quality systematic reviews of case–control or cohort studies; high quality case–control studies 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 | Nonanalytic studies, e.g. case reports, case series |
4 | Expert opinion |
Recommendation grade |
A | At least one meta-analysis, systematic review, or randomised controlled trial rated as 1++ and directly applicable to the target population or a systematic review of randomised controlled trials or a body of evidence consisting principally of studies rated as 1+ directly applicable to the target population and demonstrating overall consistency of results |
B | A body of evidence including studies rated as 2++ directly applicable to the target population and demonstrating overall consistency of results or extrapolated evidence from studies rated as 1++ or 1+ |
C | A body of evidence including studies rated as 1– or 2+ directly applicable to the target population and demonstrating overall consistency of results or extrapolated evidence from studies rated as 2++ |
D | Evidence level 2–, 3 or 4 or extrapolated evidence from studies rated as 2+ |