Table 1.

Levels of evidence.

Grade	Risk of Bias	Description
1	1++	High-quality meta-analyses, systematic reviews of RCTs, or RCTs with very low risk of bias
	1+	Well-conducted meta-analyses, systematic reviews of RCTs, or RCTs with low risk of bias
	1−−	Meta-analyses, systematic reviews, or RCTs with high risk of bias
2	2++	High-quality systematic reviews of case-control or cohort studies; high-quality case-control or cohort studies with very low risk of confounding or bias and high probability of casual relationship
	2+	Well-conducted case-control or cohort studies with low risk of confounding or bias and moderate probability of casual relationship
	2−−	Case-control or cohort studies with high risk of confounding or bias and significant risk of noncausal relationship
3		Nonanalytic studies, e.g., case reports, case series
4		Expert opinion

RCT, randomized controlled trial.