Table 1.

Levels of 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, 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
2+	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
2−	Case-control or cohort studies with a high risk of confounding or bias and a significant risk that the relationship is not causal
3	Non-analytic studies eg, case reports and case series
4	Expert opinion

RCTs, randomised controlled trials.