TABLE 1.

Definition of 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. 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+	Well‐conducted case‐control or cohort studies with a low risk of confounding or bias and a moderate 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, for example, case reports, case series
4	Expert opinion

Note: According to the Scottish Intercollegiate Guidelines Network (SIGN) grading system. ³⁰ RCT, randomized controlled trial.