Funding for this research was provided by:
National Key Research and Development Program of China (2017YFC0907400)
Text and Data Mining valid from 2018-04-17
Received: 24 December 2017
Accepted: 3 April 2018
First Online: 17 April 2018
Ethics approval and consent to participate
: The protocol used in the study was approved by the National Center for Health Statistics (NCHS) according to the Helsinki Declaration.
: The authors declare that they have no competing interests.
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