1. Department of Epidemiology and Biostatistics, School of Public Health, Hebei Medical University, Shijiazhuang
050017, Hebei Province, China
2. Library, Hebei Medical University, Shijiazhuang 050017, Hebei Province, China
3. Department of Novelty Retrieval, Institute of Hebei Medical Information, Shijiazhuang 050071, Hebei Province, China