1. Stanford University Medical Center, Stanford, California 94305.
2. Department of Biostatistics, Harvard School of Public Health, Boston, Massachusetts 02169.
3. Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02215.
4. Northwestern University Medical School, Chicago, Illinois 60611.
5. University of Minnesota, Minneapolis, Minnesota 55455.
6. Ohio State University, Columbus, Ohio 43210.