Table 2

Classification scheme for category of evidence and strength of recommendation

Category of evidence
IaEvidence from the meta-analysis of randomised controlled trials
IbEvidence from at least one randomised controlled trial
IIaEvidence from at least one controlled study without randomisation
IIbEvidence from at least one other type of quasi-experimental study
IIIEvidence from non-experimental descriptive studies, such as comparative, correlation and case–control studies
IVEvidence from expert committee reports, opinions or clinical experience of respected authorities, or both
Strength of recommendation
ADirectly based on category I evidence
BDirectly based on category II evidence or extrapolated recommendation from category I evidence
CDirectly based on category III evidence or extrapolated recommendation from category I or II evidence
DDirectly based on category IV evidence or extrapolated recommendation from category I, II or III evidence
  • This classification scheme was adopted from ‘Clinical guidelines: Developing guidelines’ by Shekelle et al (1999).