Grading scheme

The classification of recommendations and the levels of evidence for intervention studies used in this guideline are adapted from the Scottish Intercollegiate Guidelines Network Handbook (SIGN 50: a guideline developer's handbook) and summarised in the tables below.

View the latest edition of the SIGN guideline developer's handbook. 

Recommendation Grade Evidence
A
  • At least one meta-analysis, systematic review, or randomised controlled trial (RCT) that is rated as 1++, and is directly applicable to the target population, or
  • A systematic review of RCTs or a body of evidence that consists principally of studies rated as 1+, is directly applicable to the target population and demonstrates overall consistency of results, or
  • Evidence drawn from a NICE technology appraisal.
B
  • A body of evidence that includes studies rated as 2++, is directly applicable to the target population and demonstrates overall consistency of results, or
  • Extrapolated evidence from studies rated as 1++ or 1+.
C
  • A body of evidence that includes studies rated as 2+, is directly applicable to the target population and demonstrates 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+, or
  • Formal consensus
D(GPP) A good practice point (GPP) is a recommendation for best practice based on the experience of the Guideline Development Group.
Levels of evidence Types of evidence
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 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-analytical studies (for example, case reports, case series).
4 Expert opinion, formal consensus.