Funding for this research was provided by:
National Natural Science Foundation of China (no. 71073176)
National Natural Science Foundation of Liaoning Province (no.2014021099)
Received: 20 September 2018
Accepted: 9 January 2019
First Online: 22 January 2019
Ethics approval and consent to participate
: Our study is a non-interventional study by a retrospective investigation. All procedures were approved by the Ethical Committee Group of China Medical University. In this study, we never referred to any individuals’ privacy. All data in the study were elicited from Fuxin Mining Area Social Security Administration Center. The use of the data was approved by them.
: Not applicable.
: The authors declare that they have no competing interests.
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