This study was conducted between October 1990 and May 1992 at Group Health Cooperative of Puget Sound, a staff-model HMO located in western Washington State. Group Health Cooperative provides comprehensive health care to approximately 380,000 enrollees and has numerous computerized and manually maintained data bases linked by the permanent medical-history numbers assigned to each person on enrollment.18 All study procedures were reviewed and approved by the institutional review board at the HMO.