Levels of evidence
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, or RCTs with a low risk of bias
1 Meta-analyses, systematic reviews, 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 or bias and a high probability that the relationship is causal
2 Well conducted case control or cohort studies with a low risk of confounding or bias and a moderate probability
that the relationship is causal
2 Case control or cohort studies with a high risk of confounding or bias and a signifi cant risk that the relationship is
not causal
3 Non-analytic studies, e.g. case reports, case series
4 Expert opinion