1. MRC Biostatistics Unit; Institute of Public Health; Robinson Way Cambridge CB2 0SR U.K.
2. DBIM; Hôpital Saint-Louis, APHP; Paris France
3. Université Paris Diderot; Paris France
4. Inserm UMRS 717; Paris France
5. Department of Medical Statistics; London School of Hygiene and Tropical Medicine; Keppel Street London WC1E 7HT U.K.
6. Julius Center for Health Sciences and Primary Care; University Medical Center Utrecht; PO Box 85500 3508 GA Utrecht The Netherlands