Chih-Sheng Hsieh is Assistant Professor at The Chinese University of Hong Kong, China. His research interests span over labor economics, health economics, and social networks. In particular, he works on endogenous selection issues inherited in social interaction studies, and empirical network formation to accommodate important features from observed network data, such as homophily in characteristics, behaviors, as well as link dependence. Michael D. Koenig is a research associate at the Department of Economics at the University of Zurich since 2012. He holds a master's degree in economics from the University of Zurich, a master's degree in theoretical physics from the Technical University of Vienna and a diploma in applied statistics from the Seminar for Statistics at ETH Zurich. Prior to joining the University of Zurich, he was a visiting scholar at the Stanford Institute for Economic Policy Research (SIEPR) and the Department of Economics at Stanford University. Xiaodong Liu is Associate Professor at University of Colorado, Boulder. His research interests are in econometrics and applied microeconomics. His current research is focused on estimation methods in evaluating the extent of social, spatial and strategic interactions among economic agents.