1. 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
2. Division of Health Statistics, School of Public Health, Shanxi Medical University, Taiyuan, PR China
3. Department of Mathematics, San Francisco State University, San Francisco, CA, USA
4. Department of Statistics and Probability, Michigan State University, East Lansing, MI, USA