1. Beth Ann Griffin is a doctoral student and Stephen Lagakos is Professor of Biostatistics , Biostatistics Department, Harvard School of Public Health, Boston, MA 02115. This work was supported in part by National Institutes of Health training grant T32-AI007358 and research grant AI24643. The authors gratefully thank to Dr. John Gallin, Dr. Steven Holland, and Betty Marciano for providing the data used in Section 6 and the reviewer and editor for their helpful suggestions.