1. Department of Statistics, University of California, 425 Evans Hall, Berkeley, California 94720, U.S.A. pengdingpku@berkeley.edu
2. Department of Epidemiology, Harvard T. H. Chan School of Public Health, 677 Huntington Avenue, Boston, Massachusetts 02115, U.S.A. tvanderw@hsph.harvard.edu robins@hsph.harvard.edu