Table 3.

Levels of evidence [5]

Levels	Evidence
1++	High-quality meta-analyses, systematic reviews of RCT, or RCT with a very low ROB
1+	Well-conducted meta-analyses, systematic reviews, or RCT with a low ROB
1−	Meta-analyses, systematic reviews, or RCT with a high ROB
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, e.g., case reports, case series
4	Expert opinion

RCT, randomized controlled trials; ROB, risk of bias.