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

From: Selective prevention programs for children from substance-affected families: a comprehensive systematic review

Levels of evidence  
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