Department of Statistics, University of Warwick, Coventry, CV4 7AL, MRC Biostatistics Unit, Cambridge, CB2 0SR, UK, Life Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94710, Department of Biomedical Engineering, Knight Cancer Institute, Oregon Health and Science University, Portland, OR 97239-3098, USA and School of Clinical Medicine, University of Cambridge, Cambridge, CB2 0SP, UK Department of Statistics, University of Warwick, Coventry, CV4 7AL, MRC Biostatistics Unit, Cambridge, CB2 0SR, UK, Life Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94710, Department of Biomedical Engineering, Knight Cancer Institute, Oregon Health and Science University, Portland, OR 97239-3098, USA and School of Clinical Medicine, University of Cambridge, Cambridge, CB2 0SP, UK.