1. Department of Preventive Medicine, College of Medicine, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-705, Republic of Korea
2. Department of Economics, Economics Research Institute, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-705, Republic of Korea
3. Department of Nursing, College of Medicine, Chosun University, Dong-gu, Gwangju 61452, Republic of Korea
4. Department of Preventive Medicine, College of Medicine, Kyung Hee University, Dongdaemun-gu, Seoul 130-701, Republic of Korea