The current study focuses specifically on potential heterogeneity of the phenotype. Since we consider age of onset of different substances, and it is likely that the general population from which the sample was drawn contains (at least) two subpopulations, namely subjects who will never initiate use, and subjects who have already initiated or who might initiate use in the future. Therefore, two separate sets of mixture analyses were conducted, namely (1) a survival mixture analysis (SMA) to investigate potential heterogeneity regarding the risk of first use of tobacco, alcohol, cannabis, inhalants, and other substances (i.e., the category including LSD, PCP, amphetamines, and ecstasy), and (2) a latent class analysis (LCA) conducted on subjects with reported first use to examine differential patterns of first use of the same five (groups of) substances. The two sets of analyses aimed at answering different but complementary questions. The SMA addressed the risk of initiation for the five phenotypes separately. Survival mixture models permit differentiation between subjects who never use (i.e., “long-term survivors”), and subjects who have initiated use or are likely to initiate use
