1. MRC Biostatistics Unit, Cambridge Institute of Public Health, University of Cambridge, Cambridge CB2 0SR, United Kingdom;
2. Department of Biostatistics, University of Pittsburgh, Pittsburgh, Pennsylvania 15261;
3. Department of Biostatistics, Department of Genetics, University of North Carolina, Chapel Hill, North Carolina 27599;
4. Public Health Sciences Division, Fred Hutchinson Cancer Research Center, Seattle, Washington 27516