Table 1

Levels of evidence19

LevelType of evidence (based on AHCPR 1992)
IaEvidence obtained from meta-analysis of randomised controlled trials
IbEvidence obtained from at least one randomised controlled trial
IIaEvidence obtained from at least one well designed controlled study without randomisation
IIbEvidence obtained from at least one other type of well designed quasiexperimental study
IIIEvidence obtained from well designed non-experimental descriptive studies, such as comparative studies, correlation studies, and case-control studies
IVEvidence obtained from expert committee reports or opinions and/or clinical experience of respected authorities