For network evolution, friendship choices were predicted as expected, by ego outdegrees (the fewer friends ego has chosen, the more likely that another will be added), reciprocity (if alter has already chosen ego, ego is more likely to choose alter, or alter drops his or her choice), transitivity (ego is more likely to choose alters who are friends of existing friends), 3-cycles (a tendency to avoid intransitive friendship structures, that is, a chooses b chooses c chooses a), same or adjacent grade, same gender and ethnicity, as well as effects of the gender and grade of ego and alter. These effects are known alternatives to friendship formation based on drinking similarity, hence controlling for them is important. Effect directions and approximate magnitudes were similar to those obtained in other SABM studies of adolescent friendships (e.g., Burk et al., 2012; Knecht et al., 2011; Mercken et al., 2012).
