We estimate the average treatment effect (ATE) parameter and treatment on the treated (TT) for those students effectively choosing a private voucher school. With detailed individual data for both the school and the family of the student we are able to control for factors not used before in the literature. Controlling for the selection bias caused by school choice, we estimate a small ATE but a substantial TT. The paper concludes that the voucher system works to provide a better education to those that use it