In order to assess the extent to which genetic, shared environmental and non-shared environmental influences on the onset of use overlap across substances, a trivariate triangular decomposition model (also known as a Cholesky decomposition model) was fitted. The proportions of variance attributable to common vs. substance-specific sources of influence on initiation of alcohol, cigarette, and cannabis use were estimated under this model. Models were constructed based on the best-fitting univariate models and fitted in Mx using raw categorical data. Thresholds for all three initiation variables were simultaneously adjusted for all three lag times between age at first use and age at time of report. The final model was derived by testing sub-models against the full model, as described in Section 2.5.2.
