Third, we were concerned about the degree to which our interactions resulted from the prediction of drinking versus not drinking versus the quantity of alcohol consumed given drinking. We explored this question for the ages 12–14 and 15–17 where we observed our interactions. At ages 12–14, most but not all of the interactions resulted from the drinker–non-drinker dichotomy (analyzed on a scale of direct probability using a quasi-likelihood approach, not by logistic regression). At ages 15–17, approximately equal proportions of the interaction arose from the drinker–non-drinker dichotomy and from the quantity consumed among drinkers. A substantial rise in levels of alcohol use from ages 12–14 to ages 15–17 (Fig. 1) probably contributed to greater power to observe interactions in the quantity of alcohol consumed at the later age.
