Table 3  Levels of evidence according to SIGN (November 2011).

Level	Description
Source: http://www.sign.ac.uk/pdf/sign50.pdf Source (contents, abbreviations, notes): http://www.cebm.net/?o=1025
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, or 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 studies or cohort studies with a low risk of confounding or bias and a moderate probability that the relationship is causal
2−	Case-control studies 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