Funding for this research was provided by:
the Natural Science Foundation of Gansu Province (1606RJZA038)
the Scientific Study Project for Gansu Province Institutes of Higher Learning (2017B-41)
the National Natural Science Foundation of China (11661051)
the Sichuan province science and technology plan project (No.2017JY0195)
the Start-up Funding of Neijiang Normal University (16B01)
the National Statistical Scientific Research Projects (2017LZ41)
Article History
Received: 6 December 2017
Accepted: 11 April 2018
First Online: 20 April 2018
Competing interests
: The authors declare that there is no conflict of interests regarding the publication of this paper.