Secondly, analyses of the progression from opportunity to cannabis use initiation were not possible, as timing of transitions was only available as time in years, and there was not enough variation in the speed of this transition to allow for analysis (the majority of participants progressed to use within 1 year after having the opportunity to use, data available on request). Thirdly, selected covariates measured occurrence within an age range (6–13), and consequently may have occurred prior to the age of opportunity to use cannabis for a small number of individuals. Fourthly, while the prevalence of lifetime cannabis use in this sample was relatively high at 68.2% (Lynskey et al., 2012) it is important to note this estimate is consistent with previous estimates from the Australian young adult population (Australian Institute of Health and Welfare, 2014). Finally, interpretation of these analyses should be in light of the twin and sibling sample used, as there is some residual uncertainty about whether inferences from twin data have external validity with respect to what might be found in general population samples (Vitaro et
