Maximum likelihood factor analysis was conducted in SAS (42), with tests for validity in MPlus (43). The number of factors necessary to explain covariance across the items assessing RSI and PSI, which were factor-analyzed separately, was assessed using Eigenvalues and visual inspection of scree plots. A single factor satisfactorily explained the data on regular alcohol use, regular cigarette smoking and repeated cannabis use. Factor loadings ranged from0.40–0.65 (Table 1) – the factor score resulting from this analysis constituted the RSI measure. For peer substance use, 2 factors were initially extracted. However, the 1st factor explained 80% of the covariance across items, with factor loadings ranging from 0.36 to 0.75 (Table 1) – we used this factor to represent PSI. The generated factor scores for PSI and RSI were extracted and PROC REG was used to regress out the effects of age (defined as 3 dummy measures representing age at questionnaire: age less than 16 years, between 16 and 17 years and between 18 and 20 years with age greater than 21 years as the reference). The corresponding residuals, representing the
