Correspondence
Xiaorong Yang, Clinical Epidemiology Unit, Qilu Hospital of Shandong University, No. 107, Wenhua West Road, Jinan, Shandong 250012, P. R. China.
E-mail: yangxiaorong@sdu.edu.cn
Ming Lu, Clinical Epidemiology Unit, Qilu Hospital of Shandong University, No. 107, Wenhua West Road, 250012, Jinan, Shandong 250012, P. R. China.
E-mail: lvming@sdu.edu.cn