Table 2 Scottish Intercollegiate Guideline Network: levels of evidence [151]

1++

High-quality meta-analyses, systematic reviews of randomised controlled trials (RCTs), or RCTs with a very low risk of bias

1+

Well-conducted meta-analyses, systematic reviews 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 or cohort 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-analytic studies (for example, case reports and case series)

4

Expert opinion