1. School of Information, University of Michigan, Ann Arbor, MI 48109
2. Center for the Study of Complex Systems, University of Michigan, Ann Arbor, MI 48109
3. Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109
4. School of Communication, Northwestern University, Evanston, IL 60208
5. McCormick School of Engineering, Northwestern University, Evanston, IL 60208; and
6. Northwestern University Institute on Complex Systems, Evanston, IL 60208