Table 2:
Levels of Evidence | Type of Evidence |
---|---|
1a | High-quality meta-analyses, systematic reviews of RCTs, or RCTs with a very low risk of bias |
1b | Well-conducted meta-analyses, systematic reviews of RCTs, or RCTs with a low risk of bias |
1c | Meta-analyses, systematic reviews of RCTs, or RCTs with a high risk of biasb |
2a | High-quality systematic reviews of case–control or cohort studies; 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 |
2b | 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 |
3 | Case–control or cohort studies with a high risk of confounding bias or chance and a significant risk that the relationship is not causalb |
4 | Nonanalytic studies (for example, case reports, case series) |
5 | Expert opinion, formal consensus |
Note:—RCT indicates randomized controlled trial.
Adapted from Reference 3a.
Studies with a level of evidence indicated by—should not be used as a basis for making a recommendation.