1. Department of Biostatistics, Oslo Centre for Biostatistics and Epidemiology (OCBE), Institute of Basic Medical Sciences, University of Oslo , Oslo 0317 , Norway
2. Department of Cancer Genetics, Institute for Cancer Research, Oslo University Hospital , Oslo 0310 , Norway
3. Department of Medical Statistics, Faculty of Epidemiology and Population Healthy, London School of Hygiene & Tropical Medicine , London WC1E 7HT , UK