Levels of evidence
| Grade | Evidence |
| 1++ | High quality meta-analyses, systematic reviews of 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 casual |
| 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 casual |
| 2− | Case-control or cohort studies with a high risk of confounding, bias or chance and a significant risk that the relationship is not casual |
| 3 | Non analytic studies, for example, case reports, case series |
| 4 | Expert opinion |
RCT, randomised control trial.