The environment of adolescent peer drinking was derived from participant responses to questions addressing the proportion of best friends who drink. With the longitudinal design of the study, 88% (1366/1550) of participants received at least one adult SSAGA assessment at age 18 years or older. Assignment of the level of peer drinking in these participants was determined from the first adult SSAGA interview with the question “When you were 12-17, how many of your best friends used alcohol?” and the 4 possible answers of none, few, most, or all. For participants who had not reached age 18 at the last assessment, peer drinking was evaluated with the maximum value from all C-SSAGA answers to the question “How many of your best friends use alcohol?” For the primary analyses, peer drinking was dichotomized into low peer drinking (few or no best friends drink) and high peer drinking (most or all best friends drink) as done in previous studies (Kuperman et al., 2013). The four level peer drinking variable (none, few, most, or all best friends) was also investigated in secondary analyses
