1. Department of Statistical Sciences, University of Padova, Via Cesare Battisti 241/243, 35121 Padova, Italy,
2. Department of Biostatistics, University of Copenhagen, Øster Farimagsgade 5B, P.O.B. 2099, DK-1014 Copenhagen K, Denmark
3. Department of Biostatistics, University of Southern Denmark, J. B. Winsløvsvej 9 B, DK-5000 Odense, Denmark