For the network portion of the model, selection based on same alcohol onset status was of prime concern (main effect and interactions). However, as for any multivariate statistical model, validly inferring selection effects depends on controlling for correlated alternatives. The most basic such alternatives are those associated with proximity, either physical proximity (e.g., taking the same classes) or network proximity (e.g., being friends with one another’s friends). Gender and ethnicity selection effects may also be correlated with alcohol-based selection. We controlled for these effects by including same grade, same ethnicity, same gender, reciprocity (friendship choices have a tendency to be reciprocated), transitivity (friends of friends are more likely to become friends) and 3-cycles (an inverse measure of hierarchy; see Snijders et al., 2010) as predictors of friendship selection, in addition to drinking onset status for ego (the chooser) and alter (the chosen). These effects have been widely studied and are typically strong predictors of friendship formation. Fortunately, moreover, whereas specific selection effect parameters may be biased by a failure to include more fundamental effects, omission of some context effects for
