For cigarette smoking and alcohol drinking, participants were asked two questions “On how many occasions (if any) have you done the following things (“smoked cigarettes”, “drunk alcohol”) in the last 30 days?” For marijuana use, participants were asked “Have you ever taken marijuana (pot, weed, hashish, joint) in the past 30 days?” Cigarette smoking, alcohol drinking and marijuana use had the same response scale with 7 categories (never; once or twice; 3–5 times; 6–9 times; 10–19 times; 20–39 times; 40 times or more). Because participant responses were skewed, with few reporting high levels of substance use, we dichotomized the categories into a binary variable (“never” vs. “once to 40 times or more”).
