1. Department of Otorhinolaryngology-Head and Neck Surgery, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, Korea.
2. Department of Otolaryngology, Asan Medical Center, University of Ulsan College of Medicine, Seoul, Korea.
3. Department of Otorhinolaryngology, The Catholic University of Korea School of Medicine, Seoul, Korea.
4. Department of Otorhinolaryngology-Head and Neck Surgery, Konkuk University School of Medicine, Seoul, Korea.
5. Department of Otorhinolaryngology, Seoul National University Bundang Hospital, Seoul National University College of Medicine, Seongnam, Korea.
6. Department of Otorhinolaryngology, Seoul National University College of Medicine, Seoul, Korea.
7. Department of Otorhinolaryngology, Yonsei University College of Medicine, Seoul, Korea.
8. Department of Otorhinolaryngology-Head and Neck Surgery, Hanyang University College of Medicine, Seoul, Korea.
9. Department of Otorhinolaryngology-Head and Neck Surgery, Kyung Hee University School of Medicine, Seoul, Korea.
10. Department of Otolaryngology-Head and Neck Surgery, Soonchunhyang University College of Medicine, Bucheon, Korea.
11. Division of Chronic Disease Surveillance, Korea Centers for Disease Control & Prevention, Seoul, Korea.