Funding for this research was provided by:
National Natural Science Foundation of China (81402692)
Excellent Talents Fund Program of Peking University Health Science Center (BMU2017YJ002)
Received: 20 January 2018
Accepted: 19 October 2018
First Online: 26 October 2018
Ethics approval and consent to participate
: This study was a secondary analysis of the identified CHARLS public data. The Medical Ethics Committee of Peking University granted the current study exemption from review. ZZ and ZW received permission from the CHARLS team to use this.
: Not applicable.
: The authors declare that they have no competing interests.
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