1. Engineering Systems Division, MIT, Cambridge, MA 02144, USA
2. School of Information Science, University of Pittsburgh, Pittsburgh, PA 15260, USA
3. Department of Economics, UC Davis, Davis, CA 95616, USA
4. Harvard Kennedy School, Harvard University, Cambridge, MA 02144, USA
5. Department of Civil and Environmental Engineering, MIT, Cambridge, MA 02144, USA
6. Lazer Laboratory, Department of Political Science and College of Computer and Information Science, Northeastern University, Boston, MA 02115, USA