Department of Health Statistics, School of Public Health, Shanxi Provincial Key Laboratory of Major Disease Risk Assessment, Shanxi Medical University, 56 South XinJian Road, Taiyuan, 030001, China. Department of Mathematics, School of Basic Medical Sciences, Shanxi Medical University, Taiyuan, China. Shanxi Provincial Key Laboratory of Major Diseases Risk Assessment, Taiyuan, China. Department of Health Statistics, School of Public Health, Shanxi Provincial Key Laboratory of Major Disease Risk Assessment, Shanxi Medical University, 56 South XinJian Road, Taiyuan, 030001, China. yu@sxmu.edu.cn. Shanxi Provincial Key Laboratory of Major Diseases Risk Assessment, Taiyuan, China. yu@sxmu.edu.cn.