Funding for this research was provided by:
National Natural Science Foundation of China (71490732)
Ministry of Science and Technology of the People's Republic of China (2017YFC1311100)
Received: 26 October 2018
Accepted: 25 February 2019
First Online: 15 March 2019
Ethics approval and consent to participate
: The CLHLS study was approved by the Research Ethics Committee of Peking University (IRB00001052-13074), and all participants or their proxy respondents provided written informed consent.
: Not applicable.
: The authors declare that they have no competing interests.
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