A CCC model was used for analyses, which allows substance use to be conceptualized as a two-stage process incorporating initiation of substance use and progression to heavier use [23–25]. This model allows the estimation of the magnitude of the relationship between initiation and progression (by means of a beta pathway between these two stages [see Fig. 1]). If the beta coefficient is estimated to be zero, this suggests that the initiation and progression stages are entirely unrelated processes, i.e. genetic and environmental risk factors for initiation are completely independent from those for progression. If the beta coefficient is estimated to be 1, this indicates that initiation and progression are entirely overlapping dimensions with identical genetic and environmental risk factors. The 95% confidence intervals around the beta coefficient provide further information on the degree of overlap between the two stages. Lower limits closer to 0 (or below) support independent liabilities and upper limits approaching 1 provide support for identical liabilities. This model therefore provides a means of testing directly the strength of association between the initiation and progression stages for a
