Table 1 Levels of evidence according to the Scottish Intercollegiate Guidelines Network (SIGN, 1999)

Ia

Evidence from a systematic review or meta-analysis of randomized controlled trials (RCT’s)

Ib

Evidence from at least one RCT

IIa

Evidence from at least one well-designed controlled study without randomization

IIb

Evidence from at least one well-designed quasi-experimental study, such as a cohort study

III

Evidence from a well-designed non-experimental descriptive studies, such as comparative studies, correlation studies, case–control studies and case series

IV

Expert committee reports, opinions and/or clinical experience of respected authorities