1. Department of Surgery, Chungbuk National University Hospital, Cheongju, Korea.
2. Department of Surgery, Seoul National University Bundang Hospital, Seongnam, Korea.
3. Department of Anesthesiology and Pain Medicine, Seoul National University Bundang Hospital, Seongnam, Korea.
4. Division of Statistics, Medical Research Collaborating Center, Seoul National University Bundang Hospital, Seongnam, Korea.