1. Department of Statistics; London School of Economics and Political Science; London UK
2. Economic Research Unit; Cagliari Branch; Bank of Italy; Cagliari Italy
3. F.R.S.-FNRS; 5 Rue d'Egmont 1000 Brussels Belgium
4. European Center for Advanced Research in Economics and Statistics (ECARES); Solvay Business School of Economics and Management; Université libre de Bruxelles; 50 av. F.D. Roosevelt, CP 114 B1050 Brussels Belgium