Funding for this research was provided by:
The National Natural Science Foundation Program of China (No. 71671073)
Received: 27 August 2019
Accepted: 11 June 2020
First Online: 17 June 2020
Ethics approval and consent to participate
: This research used non-identifiable secondary data that are publicly accessible at the CHARLS official website. Therefore, ethical approval and consent to participate are not applicable.
: Not applicable.
: The authors declare that they have no competing interests.