Department of Health Statistics, Second Military Medical University, No. 800 Xiangyin Road, Shanghai, 200433, China. Unit of Biostatistics, Institute of Environmental Medicine, Karolinska Institutet, Nobelsväg 13, Box 210, Stockholm, 17 177, Sweden. xin.fang@ki.se. Mount Sinai St. Luke's and West Medical Center, 1111 Amsterdam Ave., New York, NY, 10025, USA. Department of Cardiology, Peking Union Medical College Hospital, Peking Union Medical College, Chinese Academy of Medical Sciences, Beijing, 100730, China. Faculty of Public Health, Hedmark University of Applied Sciences, 2411, Elverum, Norway. Innlandet Hospital Trust, Kongsvinger Hospital Division, 2226, Kongsvinger, Norway. Clinical Epidemiology and Biostatistics, School of Medical Sciences, Örebro University, 701 82, Örebro, Sweden. Department of Health Statistics, Second Military Medical University, No. 800 Xiangyin Road, Shanghai, 200433, China. hejia63@yeah.net. Unit of Biostatistics, Institute of Environmental Medicine, Karolinska Institutet, Nobelsväg 13, Box 210, Stockholm, 17 177, Sweden.