Table 4. The SIGN classification of evidence (40).

Levels of evidence	Description
1++	High-quality meta-analyses, systematic reviews of RCTs, or RCTs with very low risk of systematic error (bias)
1+	Well-conducted meta-analyses, systematic reviews of RCTs, or RCTs with low risk of systematic error (bias)
1−	Meta-analyses, systematic reviews of RCTs, or RCTs with high risk of systematic error (bias)
2++	High-quality systematic reviews of case control or cohort studies or high-quality case control or cohort studies with very low risk of systematic distortion (confounding, bias, chance) and high probability that the relationship is causal
2+	Well-conducted case control or cohort studies with low risk of systematic distortion (confounding, bias, chance) and moderate probability that the relationship is causal
2−	Case control or cohort studies with high risk of systematic distortion (confounding, bias, chance) and significant risk that the relationship is not causal
3	Non-analytical studies, e.g., case reports or case series
4	Expert opinion

RCT, randomized controlled trial