Funding for this research was provided by:
National Natural Science Foundation of China (81101803)
Tongji University (1501219143)
Text and Data Mining valid from 2019-04-04
Received: 18 November 2018
Accepted: 11 February 2019
First Online: 4 April 2019
Compliance with ethical standards
: The authors of this manuscript have no conflicts of interest.
: No ethics approval was required as this study was an economic analysis without human subjects.