Table 1 Hierarchy of levels of evidence from published papers, and grades of recommendation based on this hierarchy, as used by the Scottish Intercollegiate Guidelines Network [6]

Hierarchy of levels of evidence from published papers

   1++

High quality meta-analyses, systematic reviews of randomized 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, eg, case reports, case series

   4

Expert opinion

Grades of recommendation

   A

At least one meta-analysis, systematic review, or RCT rated as 1++ and directly applicable to the target population, or

A systematic review of RCTs or a body of evidence consisting principally of studies rated as 1+, directly applicable to the target population and demonstrating overall consistency of results

   B

A body of evidence including studies rated as 2++, directly applicable to the target population and demonstrating overall consistency of results, or

Extrapolated evidence from studies rated as 1++ or 1+

   C

A body of evidence including studies rated as 2+, directly applicable to the target population and demonstrating overall consistency of results, or

Extrapolated evidence from studies rated as 2++

   D

Evidence level 3 or 4, or

Extrapolated evidence from studies rated as 2+