Analyses were conducted using SABM for the network and alcohol onset measures (Snijders, 2001; Snijders et al., 2010) implemented in the R package RSiena (Ripley, Snijders, & Preciado, 2012). Network tie changes and behavior changes are modeled simultaneously as coevolving Markov processes. Individuals make friendship choices independently at randomly selected times. Choice probabilities are given by a multinomial logit distribution that can depend on characteristics of the actor (e.g., gender, grade, level of modeled behaviors), of individuals the actor is linked to (e.g., number or proportion of friends who drink alcohol), or of characteristics of the linkage (e.g., to an actor of same or different ethnicity). Behavior change is usually modeled similarly, but, in this study, the onset model is novel, as explained further on. Parameters associated with each predictor are estimated using a method-of-moments approach (Godambe, 1991) that involves repeatedly simulating the model-implied network and behavioral evolution, adjusting the interim parameter values at each step to improve the selected fit criterion.
