National Cancer Institute Biometric Research Branch, National Institutes of Health, 6130 Executive Blvd., MSC 7434, Bethesda, MD 20892-7434, USA. wrightge@mail.nih.gov|National Cancer Institute Biometric Research Branch, National Institutes of Health, 6130 Executive Blvd., MSC 7434, Bethesda, MD 20892-7434, USA. wrightge@mail.nih.gov