1. Department of General Surgery, Qingdao municipal hospital, Qingdao, China
2. Department of General Surgery, Affiliated Hospital of Qingdao University, Qingdao, China
3. Department of General Surgery, Yantai Yuhuangding Hospital, Yantai, China
4. Department of Epidemiology and Health Statistics, Qingdao University Medical College, Qingdao, China
5. Department of General Surgery, Zhucheng People's Hospital, Weifang, China
6. Department of General Surgery, Linzi District People's Hospital, Zibo, China