1. Department of Epidemiology, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, 2-5-1 Shikata-cho, Kita-ku, Okayama 700-8558, Japan
2. Health Service Center, Okayama University, 2-1-1 Tsushimanaka, Kita-ku, Okayama 700-0082, Japan
3. Department of Social Epidemiology, Graduate School of Medicine and School of Public Health, Kyoto University, Yoshida-konoe-cho, Sakyo-ku, Kyoto 606-8501, Japan