Michael Bretthauer1, Mette Kalager2. 1. Clinical Effectiveness Research Group, University of Oslo, Oslo, Norway; Department of Transplantation Medicine, Oslo University Hospital, Oslo, Norway; Department of Epidemiology, Harvard School of Public Health, Boston, MA, USA; Frontier Science Foundation, Boston, MA 02446, USA. Electronic address: michael.bretthauer@medisin.uio.no. 2. Clinical Effectiveness Research Group, University of Oslo, Oslo, Norway; Department of Transplantation Medicine, Oslo University Hospital, Oslo, Norway; Department of Epidemiology, Harvard School of Public Health, Boston, MA, USA.