1. Department of Obstetrics, Gynecology, and Reproductive Sciences, School of Medicine, University of California, San Francisco, CA 94143-0856.
2. Department of Obstetrics, Gynecology, and Reproductive Sciences, School of Medicine, University of California, San Francisco, CA 94143-0856; Medical Effectiveness Research Center, University of California, San Francisco, CA 94143-0856 and Department of Epidemiology and Biostatistics, School of Medicine, University of California, San Francisco, CA 94143-0856.
3. Institute of Health Economics, University of Alberta, Edmonton, AB, T5J 3N4, Canada.