Department of Epidemiology and Health Statistics, School of Public Health, and Hebei Province Key Laboratory of Environment and Human Health, Hebei Medical University, Shijiazhuang, PR China.|Division of Health Statistics, School of Public Health, Shanxi Medical University, Taiyuan, PR China.|Department of Mathematics, San Francisco State University, San Francisco, CA, USA.|Division of Health Statistics, School of Public Health, Shanxi Medical University, Taiyuan, PR China.|Division of Health Statistics, School of Public Health, Shanxi Medical University, Taiyuan, PR China.