1. Department of Operational Research and Financial Engineering; Princeton University; Princeton 08540 NJ U.S.A.
2. Administrative Data Research Centre for England and Institute of Child Health; University College London; London NW1 2DA U.K.
3. Centre for Biostatistics and Arthritis Research UK Epidemiology Unit; The University of Manchester; Manchester M13 9PL U.K.
4. Department of Mathematics; The University of York; York YO10 5DD U.K.