Table 1.

Scottish Intercollegiate Guideline Network levels of evidence

1++	High-quality meta-analysis, systamatic reviews of RCTs, or RCTs with a very low risk of bias
1+	Well-conducted meta-analysis, systematic review of RCTs or RCTs with a low risk of bias
1-	Meta-analyses, systematic reviews of RCTs or RCTs with a high risk of bias
2++	High-quality systematic reviews of case–control or cohort studies High-quality case–control studies with a very low risk of confounding bias, or chance and a high probability that the relationship is causal
2+	Well-conducted case control or cohort studies with a low risk of confounding, bias or chance, and a moderate probability that the relationship is causal
2-	Case-control or cohort studies with a high risk of confounding, bias or chance, and a significant risk that the relationship is not causal
3	Non-analytical studies—for example, case series, case reports, qualitative
4	Expert opinion