1. School of Information, University of Michigan, Ann Arbor, Michigan 48109;
2. Department of Economics, School of Economics and Management, Tsinghua University, Beijing 100084, China;
3. School of Computing and Information, University of Pittsburgh, Pittsburgh, Pennsylvania 15260;
4. School of Computer Science, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213;
5. Google LLC, Mountain View, California 94043