Table 1 Levels of evidence according to SIGN

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

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 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