1. Arnošt Komárek is Research Assistant, and Emmanuel Lesaffre is Professor, Katholieke Universiteit Leuven, Biostatistical Centre, Kapucijnenvoer 35, B–3000, Leuven, Belgium (E-mail addresses: and . Joan F. Hilton is Associate Professor in Residence, University of California San Francisco, Department of Epidemiology and Biostatistics, 500 Parnassus Avenue, CA 94143–0560, San Francisco, CA .