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.