Table 1 Categories of evidence

Ia

Evidence from meta-analysis of randomised controlled trials

Ib

Evidence from at least one randomised controlled trial

IIa

Evidence from at least one controlled study without randomisation

IIb

Evidence from at least one other quasi-experimental study

III

Evidence from descriptive studies, such as comparative studies, correlation studies and case-control studies

IV

Evidence from expert committee reports or opinions or clinical experience pf respected authorities or both