Funding for this research was provided by:
National Natural Science Foundation of China (81902247)
Received: 16 September 2019
Accepted: 11 October 2020
First Online: 10 November 2020
Ethics approval and consent to participate
: CHARLS was approved by the Ethical Review Committee (IRB) at Peking University, Beijing, China. All the participants signed informed consent voluntarily.
: Not applicable.
: None of the authors have any competing interests.