Inspection of the full tetrachoric correlation matrix revealed sibling problem alcohol use and sibling drug use were collinear as were friend alcohol and drug use and school peer alcohol and drug use (polychoric correlations >0.65). Thus we created combined sibling alcohol/drug, friend alcohol/drug use and school peer alcohol/drug use variables. The full correlation matrix is available upon request. All analyses were computed using the SURVEYLOGISTC procedure in SAS v.9.0 which adjusts error variance in non-independent observations.
