1. IBM T.J. Watson Research Center, 1101 Kitchawan Road, Yorktown HeightsNew York 10598U.S.A.
2. Biostatistics Program, College of Public Health and Human Sciences, Oregon State University, CorvallisOregon 97331U.S.A.
3. Department of Statistics, University of South Carolina, ColumbiaSouth Carolina 29208U.S.A.