___________________________________Ernest Pascarella ii ACKNOWLEDGMENTS As a conclusion of a chapter of my life, this dissertation becomes a reality with the kind support and the help of many individuals. I would like to express my sincere gratitude to all of them. Foremost, I would like to express my deepest appreciation to my committee chair, Professor David Bills. His brilliant guidance, wisdom, patience, understanding, and encouragement pushed me forward to complete this project. Without this, this dissertation would never have been possible. I am also highly indebted to my committee members, Professor Last but not least, I would like to express my gratitude to my families in both China and U.S., especially my beloved and supportive wife, Jinyu, whose unselfish support and persistent belief help me survive all the stress and not let me give up throughout the years in completion of this dissertation iv ABSTRACT Using data from the National Longitudinal Study of Adolescent to Adult Health (Add Health), I investigate the influence of adolescent friendship network on the likelihood of college enrollment, and whether and how this influence is affected by stratification factors (e.g., gender, race/ethnicity, and socioeconomic status). However,there is a challenge in evaluating this influence process since adolescents usually nonrandomly select their friends. A selection process needs to be taken into consideration simultaneously with the influence process of adolescents' friendship network on their likelihood of college enrollment. Previous research on peer effects has methodological issues and limitations. Traditional methods (e.g., multivariate regression, multilevel modeling, or propensity score matching) using limited data (e.g., cross-sectional) andmeasures of friendship network (e.g., one best friend) could not solve the problem of integrating selection process and influence process in one model. In addition, the dyadic and triadic (or even higher level) dependency among friends in the network makes it more difficult to estimate selection and influence processes using traditional methods.To address these concerns, I employ longitudinal network analysis with stochastic actor-based models (SABMs) to account for the influence of friendship network on adolescent college enrollment when simultaneously considering the selection of friendship. The co-evolution model of network dynamics (selection) and behavioral dynamics (influence) also addresses the problem of endogeneity between network change and behavioral change. However, the co-evolution model requires network data and behavioral data measured in multiple time points, so in the first stage of this research, I generate the predicted probability of college enrollment at three time points of Add Health using traditional logistic regression. Then in the second stage of this research, I use the transformed likelihood of college enrollment, a statistical artifact, as the behavior variable in the co-evolution model to examine how the likelihood of college enrollment