1. Department of Biostatistics and Epidemiology, Rutgers School of Public Health , 683 Hoes Lane West , Piscataway, New Jersey 08854, U.S.A. l.luo@rutgers.edu
2. Division of Biostatistics, University of California, Berkeley , 2121 Berkeley Way , Berkeley, California 94720, U.S.A. jingshenwang@berkeley.edu
3. Department of Statistics, North Carolina State University , 2311 Stinson Drive , Raleigh, North Carolina 27695, U.S.A