1. Department of Family Medicine; Chung-Ang University Medical Center; Seoul Korea
2. Department of Family Medicine; Yonsei University Graduate School; Seoul Korea
3. Department of Obstetrics & Gynecology; Chung-Ang University Medical Center; Seoul Korea
4. Department of Physiology, College of Medicine; Chung-Ang University; Seoul Korea
5. Department of General Practice, Erasmus MC; University Medical Center Rotterdam; Rotterdam The Netherlands
6. Department of Applied Statistics; Chung-Ang University; Seoul Korea
7. Department of Family Medicine, Severance Hospital; Yonsei University College of Medicine; Seoul Korea