1. Department of Anesthesiology and Pain Medicine, Seoul National University Hospital, Seoul National University College of Medicine, Seoul, Korea.
2. Health Insurance Policy Research Institute, National Health Insurance Service, Wonju, Korea.
3. Department of Information Statistics, Andong National University, Andong, Korea.
4. Department of Statistics, College of Natural Sciences, Kyungpook National University, Daegu, Korea.
5. Department of Surgery, Seoul National University Hospital, Seoul National University College of Medicine, Seoul, Korea.