Close friend substance use was assessed from the Home Environment section of SSAGA-IV. Four items, adapted from the FinnTwin studies (Rose and Dick, 2004), were queried to participants pertaining to the prevalence of 1) cigarette smoking, 2) alcohol use, 3) marijuana use, and 4) other illicit drug use among the participants’ best friends. For instance, participants were asked “how many of your best friends smoke?” and responded to the question on a four-point scale: “none of them” (1), a few of them (2), most of them (3), and “all of them” (4). If participants were over the age of 18, they were instead asked to report on the prevalence of substance use among their close friends retrospectively, between the ages of 12–17. Since data from adults were retrospective, specifying close friend substance use as a time-varying covariate would not have been appropriate for the full sample. Hence, only the data from baseline assessment were used for the analysis. A composite score was used.
