1. MRC Biostatistics UnitUniversity of Cambridge Cambridge UK
2. National Institute of Allergy and Infectious Diseases Baltimore Maryland
3. Departments of Medicine and EpidemiologyJohns Hopkins University Baltimore Maryland
4. Department of Population MedicineHarvard Pilgrim Health Care Institute and Harvard Medical School Boston Massachusetts
5. Department of BiostatisticsHarvard T. H. Chan School of Public Health Boston Massachusetts