The second variable selleck chem is ��current level of smoking.�� To conform to best practices for SIENA (Simulation Investigation for Empirical Network Analysis) coevolutionary modeling (Ripley & Snijders, 2010), we compute smoking amount as a log transformation of the average number of cigarette smoked per day: 1 �C ln ([number of smoking occasions during past 30 days] �� [number of cigarettes per occasion] / 30). The frequency is based on a linear interpolation of the seven-category frequency scale presented in the survey, so that smoking amounts were comparable across categories. The transformed values are then rounded to the nearest integer. The transformation and rounding are done to keep the scale between 0 and 10, to smooth the distribution, and to allow for more transitions at the lower end of the scale, where we believe the most important changes are occurring in these data (Miles & Shevlin, 2000).

To capture changes in the amount of smoking we calculated this measure at Wave I and Wave II. Analysis Approach Stochastic, Actor-Oriented Models Stochastic, actor-oriented models of the sort estimated with the SIENA package (Snijders, Steglich, Schweinberger, & Huisman, 2007) allow researchers to model relationships between changes in social network structure and individuals�� attitudes and behaviors. Technical specifications and general introductions to these models can be found elsewhere (Snijders, 2005, 2006, 2009; Snijders et al., 2007; Steglich, Snijders, & Pearson, 2010).

In brief, two conditional models, estimated simultaneously, use structural and behavioral network characteristics to predict whether an adolescent will form or maintain a friendship tie (network model) or change their smoking behavior (behavior model). The network model includes parameters that evaluate the effect of network structure at Wave I on network structure at Wave II and the effect of individual attributes and behaviors on structure (social selection mechanisms). The behavior model includes parameters that evaluate the effect of behavior (or other individual attributes) at Wave I on behavior at Wave II and the effect of structure on behavior (social influence mechanisms). We apply these coevolutionary models to investigate the relationship between high school friendship networks and two smoking behaviors.

The network component of the coevolutionary model estimates the impact of friendship on smoking Entinostat behavior (influence), whereas the behavior component estimates the impact of smoking behavior on friendship choices (selection). Missing values are replaced by the sample mean, but are not used for parameter estimation (Huisman & Snijders, 2003). Influence Processes The influence part of the model allows us to investigate how an adolescent��s friends impact their smoking behaviors.