Funding for this research was provided by:
National Natural Science Foundation of China (71774005)
Received: 22 July 2019
Accepted: 8 July 2020
First Online: 14 July 2020
Ethics approval and consent to participate
: Ethical approval for this study was not necessary because it was based exclusively on publicly available data, CHARLS, and the study subjects were not directly approached.
: Not applicable.
: The authors declare that they have no competing interests.