1. Department of Disease Control, Center for Disease Control and Prevention in Northern Theater Command, Shenyang, People's Republic of China
2. Biostatistics Department, University of Michigan, Ann Arbor, MI 48109, USA
3. China National Clinical Research Center for Neurological Diseases, Beijing Tian Tan Hospital, No. 119, South 4th Ring Road West, Fengtai District, Beijing, People's Republic of China