Funding for this research was provided by:
National Natural Science Foundation of China (71671056)
Ministry of Human Resources and Social Security (19YJA790035)
the National Statistical Science Research Projects of China (2019LD05)
Article History
Received: 2 August 2019
Accepted: 7 December 2019
First Online: 14 December 2019
Compliance with ethical standards
:
: The authors declare that they have no conflict of interest.