1. Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Nobels väg 12A, Stockholm 171 65, Sweden
2. Institute of Medical Sciences, University of Aberdeen, Foresterhill, Aberdeen AB25 2ZD, UK
3. Department of Epidemiology and Health Statistics, The School of Public Health, Fujian Medical University, Xuefu North Road 1, University Town, Fuzhou 350122, China