Table 1

Revised grading system for levels of evidence in evidence-based guidelines

GradeEvidence
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 or 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

  • RCT, randomised controlled trial.