1. S. M. Blower, Department of Epidemiology and Biostatistics and Department of Medicine, University of California, San Francisco, Box 1347, San Francisco General Hospital, San Francisco, CA 94143-1347, USA.
2. P. M. Small, Department of Medicine, Stanford Medical School, Stanford, CA 94305, USA.
3. P. C. Hopewell, Department of Medicine, University of California, San Francisco, San Francisco General Hospital, San Francisco, CA 94143-1347, USA.