These selection procedures yielded a sample of 10,330 subjects at the Phase II assessment. Children and adolescents (n = 2,537) and adults without genetic information on GABRA2 (n = 4,853) were eliminated. Because risk for alcohol dependence associated with GABRA2 has been found to be weaker among adolescents relative to adults (Dick et al. 2006a), we conducted parallel analyses with an age-restricted sample of 25 or older. In addition, we ran analyses excluding lifetime non-drinkers, for whom the effects of GABRA2 genotype are also likely to be weak. Because dropping these cases did not affect our results, we retained the full sample. Respondents with genotype information were primarily from multiplex (i.e. densely affected) families and control families. Thus, families containing only one respondent with alcohol dependence were underrepresented in the genotyped subsample. Observations were also deleted if there were missing data on other study variables (n = 365).
