Another important feature of the model is that it is uniquely suited to analysis of data from adolescent samples, some of whom may have yet to engage in substance use but who will go on to develop problem use. The position of those yet to initiate on the liability distribution of progression is unknown. These individuals are treated as a special case of missing data on the measure of progression [23]. Given the likelihood of an association between age and level of substance use, an age correction was used which adjusts the threshold for each twin according to their age on the distribution of liability to substance use. The threshold is modelled as a simple linear function: ti = t + ageita, where t is the population baseline threshold (for individuals of age zero), ta models the regression of the threshold on age and agei is the age in years of the individual i at assessment [23]. The thresholds were also allowed to vary according to gender. However, there was no significant difference between models with and without a gender
