Within-Family Program E ff ect Estimators: The Impact of Oportunidades on Schooling in Mexico 1 Susan W. Parker, Petra E. Todd, Kenneth I. Wolpin August 7, 2006 1 This research was supported by a grant from the Mellon Foundation/Population Studies Center (PSC)/University of Pennsylvania to Parker, Todd and Wolpin. Parker is a Profesora/Investigadora, Di- visión de Economía, CIDE, Mexico City. Todd is an Associate Professor of Economics and a PSC Research Associate at the University of Pennsylvania. Wolpin is a Professor of Economics and a PSC Research Asso- caite at the University of Pennsylvania. We are grateful to Oportunidades for permission to use these data and to Iliana Yachine for her help in interpreting the data.