1. Department of Anesthesiology and Pain Medicine, Seoul National University Hospital, Seoul National University College of Medicine, Seoul, Korea.
2. Department of Anesthesiology and Pain Medicine, Ewha Womans University Seoul Hospital, Seoul, Korea.
3. Department of Information Statistics, Andong National University, Andong, Korea.
4. Department of Statistics, College of Natural Sciences, Kyungpook National University, Daegu, Korea.