Table 1 The Scottish Intercollegiate Guidelines Network (SIGN) grading system 1999–2012
Level of evidence | Study design and methodological quality |
|---|---|
Level 1++ | High quality meta-analyses, systematic reviews of RCTs, or RCTs with a very low risk of bias |
Level 1+ | Well-conducted meta-analyses, systematic reviews, or RCTs with a low risk of bias |
Level 1− | Meta-analyses, systematic reviews, or RCTs with a high risk of bias |
Level 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 |
Level 2+ | Well-conducted case control or cohort studies with a low risk of confounding or bias and a moderate probability that the relationship is causal |
Level 2− | Case control or cohort studies with a high risk of confounding or bias and a significant risk that the relationship is not causal |
Level 3 | Non-analytic studies, e.g. case reports, case series |
Level 4 | Expert opinion |