1. Group of Biostatistics; International Agency for Research on Cancer; Lyon Cedex 08 69372 France
2. Division of Cancer Treatment and Diagnosis; Biometric Research Program; National Cancer Institute; Rockville 9609 Maryland
3. Group of Molecular Mechanisms and Biomarkers; International Agency for Research on Cancer; Lyon Cedex 08 69372 France
4. Faculty of Medicine and Health; University of Leeds; Leeds LS2 9JT UK