Prevalence rates for lifetime substance experimentation, repeated-use and SUDs, as well as odds-ratio estimates were computed using SAS® (SAS Institute Inc., 1987). Chi-square analyses and Fisher’s exact tests were used to compare experimentation, repeated-use and SUD rates by gender after accounting for the non-independence in the data by creating a weighting scheme based on the number of individuals in a family unit. Individuals from a family of complete twin pairs were given a weight of 0.50 while individuals from an incomplete twin pair were given a weight of 1.0 (Young et al., 2002). This provided a conservative estimate of gender and group differences in the sample. Weighted and unweighted results are provided in the tables. Log-linear analyses were utilized to assess the propensity to repeatedly use multiple substances or to be diagnosed with multiple SUDs. The odds in favor of developing a lifetime diagnosis of a SUD as opposed to a lifetime report of either abstinence or some form of non-problematic use (i.e., defined here as experimentation or repeated-use without an abuse or dependence diagnosis) 5 years later were determined
