 Sample size calculations: Due to the flexibility of the modelling approach taken i.e., the number of parameters allowed to vary, there are a number of different scenarios that can be tested to determine an adequate sample size. The primary interest is in differences in drinking over time between groups who were supplied or not supplied alcohol. Thus, regression analysis will address drinking on a dummy-coded parental supply variable. It is hypothesised that parental supply of alcohol would alter the trajectory of harmful drinking by a regression coefficient of 0.2. This regression coefficient is equivalent to a medium effect size difference between groups, a result which is not unexpected given past research. It was assumed that attrition would result in around 20 missing data on each subsequent measurement occasion. Based on simulation studies, and factoring in potential mediators and moderators, 600 children are required at a minimum. However, as small cell sizes for some associations may affect power, a final sample of 1800 children and parents has been recruited. 