Table 1

Key to evidence statements

1++High-quality meta-analyses, systematic reviews of randomised controlled trials (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 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
3Non- analytic studies, for example, case reports, case series
4Expert opinion
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