1. College of Information Studies, University of Maryland, College Park, Maryland 20742;
2. School of Information, University of Michigan, Ann Arbor, Michigan 48109;
3. Department of Economics, School of Economics and Management, Tsinghua University, Beijing 100084, China;
4. Department of Computational Medicine and Bioinformatics, University of Michigan, Ann Arbor, Michigan 48109;
5. School of Computer Science and Technology, Shandong University, Qingdao 266237, China