1. Biometry Research Group; National Cancer Institute; 9609 Medical Center Drive Rockville 20850 MD U.S.A.
2. University of Maryland, Baltimore County; Baltimore MD U.S.A.
3. Department of Biological Statistics and Computational Biology; Cornell University; Ithaca 14853 NY U.S.A.
4. Department of Biostatistics; Gillings School of Global Public Health, University of North Carolina-Chapel Hill; Chapel Hill 27599 NC U.S.A.