Andrew Gelman1, Christian P Robert2. 1. Departments of Statistics and Political Science, Columbia University, New York, NY 10027; 2. Centre de Recherche en Mathématiques de la Décision, Université Paris-Dauphine, 75775 Paris Cedex 16, France; andDepartment of Statistics, University of Warwick, Coventry, Coventry CV4 7AL, United Kingdom xian@ceremade.dauphine.fr.