1. Port Jefferson, New York 11777, USA
2. Harris School of Public Policy, University of Chicago, Chicago, Illinois 60637, USA;
3. Microsoft Research, New York, NY 10012, USA
4. Department of Information Science, Cornell University, Ithaca, New York 14853, USA;
5. Google Research, Cambridge, Massachusetts 02124, USA;
6. Department of Computer and Information Science, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA;