Table 5.1.4 Hierarchy of evidence
| Level of evidence | Type of evidence |
|---|---|
| 1 | High 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 |
| 3 | Non-analytic studies (for example, case reports, case series) |
| 4 | Expert opinion, formal consensus, and policy documents |
| Guidelines | Guidelines set by clinical groups (for example, NICE, BSG, AUGIS)—see quality assessment for an appraisal of these |