1. Department of Statistics, School of Economics, Xiamen University, Xiamen, People's Republic of China
2. Key Laboratory of Econometrics, Ministry of Education, Xiamen University, Xiamen, People's Republic of China
3. Department of Biostatistics, Yale School of Public Health, New Haven, CT, USA
4. The Wang Yanan Institute for Studies in Economics, Xiamen University, Xiamen, People's Republic of China