1. Department of Biostatistics and Epidemiology, University of Oklahoma Health Sciences Center, 801 NE 13th Street, Oklahoma City, Oklahoma 73126-0901, U.S.A
2. Département de mathématiques et de statistique, Université de Montréal, 2920 chemin de la Tour, Montréal, Québec H3C 3J7, Canada
3. Department of Mathematics and Statistics, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba R3B 2E9, Canada