Affiliation: | A Department of Biostatistics, School of Public Health, University of California, Los Angeles, CA 90095, USA E-mail: gangli@sunlab.ph.ucla.edu A2 Department of Mathematics, University of North Carolina, Charlotte, NC 28223, USA A3 Department of Social Statistics, Cornell University, Ithaca, NY 14853, USA E-mail: mtw1@cornell.edu |