1. School of International Trade and Economics, University of International Business and Economics, Beijing, 100029, China;
2. International School of Economics and Management, Capital University of Economics and Business, Beijing 100070, China;
3. Department of Statistics, Texas A&M University, College Station, TX 77840;
4. Center for Statistical Research and Methodology, U.S. Census Bureau, 4600 Silver Hill Road, Washington, DC 20233