Funding for this research was provided by:
National Natural Science Foundation of China (11801576)
Fundamental Research Funds for the Central Universities (South-Central University for Nationalities (CZQ19010)
National Statistical Science Research Project of China (2019LY59)
Article History
Received: 3 October 2020
Revised: 4 March 2021
Accepted: 18 May 2021
First Online: 3 June 2021