Cox proportional hazards modeling using SAS PROC PHREG (SAS, 2011), with the COVSANDWICH (AGGREGATE) option to adjust for correlated familial data, was used to study the effect of predictor variables on the probability of alcohol initiation. Use of Cox proportional hazards modeling also accounts for potential variable confounds and reports the unique contribution of each to the model. All model assumptions were checked for violations of the proportional hazard assumption and overall model adequacy (Box-Steffensmeier & Jones, 2004). As a preliminary step, single-predictor models were used to determine each predictor's individual effect on the likelihood of alcohol initiation. The main analysis considered five multi-predictor Cox proportional hazards models. Model 1, our initial reference model, contained the five factors from our previous study (Kuperman et al., 2013): 1) most best friends drank (MBFD: Yes versus No); 2) number of CD symptoms (0 to 15); 3) YSR externalizing score (0 to 64); 4) YSR social problems score (0 to 22); and 5) member of a high-risk AD family (Yes versus No). Model 2 removed any non-significant predictors from Model 1. Model 3
