Funding for this research was provided by:
the Humanities and Social Sciences Foundation for the Youth Scholars of Ministry of Education of China (15YJCZH066)
the Science and Technology Plan Project of Hunan Province (2016TP1020)
the Science and Technology Plan Project of Hengyang City (2017KJ183)
Hunan Provincial Natural Science Foundation of China (2018JJ4024)
the Scientific Research Fund of Hunan Provincial Education Department (17A030)
the 2018 National Statistical Science Research Project of China (2018LY05)
Article History
Received: 29 July 2018
Accepted: 7 November 2018
First Online: 23 November 2018
Competing interests
: The authors declare that they have no competing interests.