Table 5.1.4 Hierarchy of evidence

Level of evidenceType of evidence
1High quality or well conducted meta-analyses, systematic reviews of randomised control trials (RCTs), or RCTs with a low risk of bias and direct topic relevance
2+High quality or well conducted case-control or cohort studies with a low risk of confounding, bias or chance and a good probability that the relationship is causal; RCTs without direct topic relevance
2−RCTs, case-control, cohort studies, or surveys with a risk of confounding bias, or chance that the relationship is not causal
3Non-analytic studies (for example, case reports, case series)
4Expert opinion, formal consensus, and policy documents
GuidelinesGuidelines set by clinical groups (for example, NICE, BSG, AUGIS)—see quality assessment for an appraisal of these