Optimal Bandwidth Choice for the Regression Discontinuity Estimator∗ Guido Imbens† and Karthik Kalyanaraman‡ First Draft: June 2008 This Draft: February 2009 Abstract We investigate the problem of optimal choice of the smoothing parameter (bandwidth) for the regression discontinuity estimator. We focus on estimation by local linear regression, which was shown to be rate optimal (Porter, 2003). Inv