Department of Statistics, University of Georgia, 101 Cedar Street, Athens, 30602, USA.|Department of Statistics, University of Illinois at Urbana-Champaign, 725 South Wright Street, Champaign, 61820, USA.|Department of Statistics, Harvard University, One Oxford Street, Cambridge, 02138, USA.|Department of Statistics, Harvard University, One Oxford Street, Cambridge, 02138, USA.|Department of Statistics, University of Georgia, 101 Cedar Street, Athens, 30602, USA.|Department of Statistics, University of Georgia, 101 Cedar Street, Athens, 30602, USA. pingma@uga.edu.