1. Section of Biostatistics, Department of Public Health, University of Copenhagen , Øster Farimagsgade 5, 1353 Køpenhavn, Denmark
2. Department of Medical Epidemiology and Biostatistics, Karolinska Institutet , Nobels väg 12A, Stockholm 17177, Sweden
3. Biostatistics Research Branch, National Institute of Allergy and Infectious Diseases , Bethesda, 5601 Fishers Lane, Rockville, MD 20892, USA
4. Section of Biostatistics, Department of Public Health, University of Copenhagen , Copenhagen, Denmark