1. Department of Statistics, London School of Economics, Houghton Street, London WC2A 2AE, U.K
2. Department of Data Sciences and Operations, University of Southern California, 3670 Trousdale Parkway, Los Angeles, California 90089, U.S.A
3. Institute of Statistics and Big Data, Renmin University of China, 59 Zhongguancun Street, Beijing 100872, China