In wave 3 of data collection, participants were asked to report their average monthly alcohol consumption using a life history calendar interview, which improves the accuracy of retrospective recall (Freedman et al. 1988). These data were then combined into ages that correspond to meaningful developmental age ranges (as in previous studies of this sample, e.g. Kendler et al. 2008a) and extending into adulthood: ages 12–14, 15–17, 18–21, 22–25, 26–29, and 30–33 years, which will be referred to as epochs 1–6, respectively. Abstainers were assigned values of 0 (see Discussion). To each average consumption score, 1 was added, followed by a natural log-transformation to adjust for non-normality. Thus, the lowest possible log-transformed score was 0, corresponding to abstainers.
