1. School of Mathematical Sciences, Queen Mary; University of London; London E1 4NS UK
2. Institute of Statistical Research and Training; University of Dhaka; Dhaka-1000 Bangladesh
3. Center for Clinical Epidemiology; St Luke's International University; 3-6-2 Tsukiji Chuo-ku Tokyo 104-0045 Japan
4. Department of Mathematics; King's College London; Strand London WC2R 2LS UK
5. Roche Pharmaceutical Research and Early Development, Pharmaceutical Sciences, Roche Innovation Center Basel; F. Hoffmann-La Roche Ltd; Grenzacherstrasse 124 4070 Basel Switzerland