Received: 5 March 2021
Accepted: 26 July 2021
First Online: 6 September 2021
: All participants joined CHARLS voluntarily and signed a consent form before participation. The original CHARLS was approved by the Ethical Review Committee (IRB) at Peking University in June 2008 (IRB00001052–11015).
: Not applicable.
: The authors declare that they have no competing interests.