1. Departments of Anesthesia and Pain Medicine, Sung Kyun Kwan University School of Medicine, and Seoul National University School of Medicine, and Department of Mathematics, Hanyang University College of Natural Sciences, Seoul, Korea
2. Department of Anesthesiology, Sung Kyun Kwan University School of Medicine, Seoul, Korea.
3. Department of Mathematics, Hanyang University College of Natural Sciences, Seoul, Korea.
4. Department of Anesthesia and Pain Medicine, Seoul National University School of Medicine, Seoul, Korea.