1. Roche Products Ltd, 6 Flacon Way, Shire Park, Welwyn Garden City, AL7 1TW, UK
2. F. Hoffmann-La Roche Ltd, Basel, Switzerland
3. F. Hoffmann-La Roche Ltd, Boulogne-Billancourt Cedex, France
4. Department of Biostatistics, University of Liverpool, Liverpool, UK