Funding for this research was provided by:
Shandong Province Natural Science Foundation of China (ZR2016GQ02)
Young Scientists Fund of the National Natural Science Foundation of China (71704143)
Text and Data Mining valid from 2019-10-29
Received: 28 July 2019
Accepted: 30 September 2019
First Online: 29 October 2019
Ethics approval and consent to participate
: The data of this study were from China Health and Retirement Longitudinal Study (CHARLS), which obtained ethical approval from Ethical Review Committee at Peking University in January 2011.
: Not applicable.
: The authors declare that they have no competing interests.