Find the best available evidence

Levels of evidence

Level	Type of evidence
Ia	Evidence obtained from a systematic review of randomised controlled trials
Ib	Evidence obtained from at least one randomised controlled trial
IIa	Evidence obtained from at least one well-designed controlled study without randomisation or a poor quality RCT
IIb	Evidence obtained from at least one other type of well-designed quasi-experimental study
III	Evidence obtained from well-designed non-experimental descriptive studies, such as comparative studies, correlation studies and case studies
IV	Evidence obtained from expert committee reports or opinions and /or clinical experience of respected authorities e.g. from the Delphi questionnaire