1. Department of Neurosurgery, National Health Insurance Service Ilsan Hospital, Yonsei University College of Medicine, Ilsan 10444, Republic of Korea
2. Department of Neurosurgery, Gil Medical Center, Gachon University College of Medicine, Incheon 21565, Republic of Korea
3. Research Institute, National Health Insurance Service Ilsan Hospital, Yonsei University College of Medicine, Ilsan 10444, Republic of Korea
4. Department of Statistics, Korea University, Seoul 02841, Republic of Korea