The survival mixture analysis provided interesting results not only with respect to the role of the CHRNA5/B4/A3 gene cluster but also with respect to the structure of the risk to engage in the use of different substances during adolescence. For the phenotypes with a higher reported prevalence of first use (tobacco, alcohol, and cannabis) the best fitting models were 2-class models, which discriminate between reported (or potential future) users and individuals who have not reported first use and are considered to be unlikely users in the future. Likewise, the data indicated that the risk to engage in using these substances is proportional rather than constant during adolescence. For the phenotypes inhalants and other substances, the 2-class proportional hazard model and a survival model constraining the risk of using inhalants or other substances (a group including LSD, PCP, amphetamines, and ecstasy) to be constant across the age range emerged as the best fitting model for these phenotypes. The lack of clearly detecting two classes might be due to the much lower prevalence of reported first use in the Add Health sample.
