1. Department of Biostatistics, Harvard School of Public Health, 655 Huntington Avenue, Boston, Massachusetts 02115, U.S.A.
2. Department of Biostatistical Science, Dana Farber Cancer Institute, 44 Binney Street, Boston, Massachusetts 02115, U.S.A.
3. Limburgs Universitair Centrum, Universitaire Campus, B‐3590 Diepenbeek, Belgium