Table 2:

Levels of evidence for intervention studies^a

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.

^aAdapted from Reference 3a.

^bStudies with a level of evidence indicated by—should not be used as a basis for making a recommendation.