1. Biostatistics UnitGroup Health Research Institute Seattle, Washington U.S.A.
2. Department of BiostatisticsUniversity of Washington, School of Public Health Seattle, Washington U.S.A.
3. Department of Biostatistics and Computational BiologyUniversity of Rochester Rochester, New York U.S.A.
4. Department of Statistics, The Wharton SchoolUniversity of Pennsylvania Center for Pharmacoepidemiology Research and Training, University of Pennsylvania Philadelphia, Pennsylvania U.S.A.