Table 1.

Levels of evidence for intervention studies⁵

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 or RCTs with a low risk of bias
1-	Meta-analyses, systematic reviews or RCTs with a high risk of bias*
2++	High-quality systematic reviews of case-control or cohort studies. High-quality case-control or cohort studies with a very low risk of confounding or bias and a high probability that the relationship is causal. Interrupted time series with a control group: (i) there is a clearly defined point in time when the intervention occurred; and (ii) at least three data points before and three data points after the intervention
2+	Well-conducted case-control or cohort studies with a low risk of confounding or bias and a moderate probability that the relationship is causal. Controlled before-after studies with two or more intervention and control sites
2-	Case-control or cohort studies with a high risk of confounding or bias and a significant risk that the relationship is not causal. Interrupted time series without a parallel control group: (i) there is a clearly defined point in time when the intervention occurred; and (ii) at least three data points before and three data points after the intervention. Controlled before-after studies with one intervention and one control site
3	Non-analytic studies (e.g. uncontrolled before-after studies, case reports, case series)
4	Expert opinion. Legislation

RCT, randomised controlled trial.

^*Studies with an evidence level of ‘1-’ and ‘2-’ should not be used as a basis for making a recommendation.