1. Department of Clinical Biostatistics, Graduate School of Medicine, Kyoto University, Yoshida Konoe-cho Sakyo-ku, Kyoto 606-8501, Japan
2. Department of Epidemiology, University of North Carolina, 2105F McGavran-Greenberg Hall, Chapel Hill, North Carolina 27599, USA
3. Department of Biostatistics, University of North Carolina, 3103B McGavran-Greenberg Hall, Chapel Hill, North Carolina 27599, USA