"You can't just look at the plans and grade them as `A,' `B' or `C,' " said Patricia Powers, executive director of the Pacific Business Group on Health, an influential health care purchasing cooperative formed by major California employers. Her group was one of two major employer groups-the other was the California Public Employees Retirement System-that participated in the study. Other coalition members included the 21 HMOs and affiliated medical groups.

The Medstat Group looked at medical data for 1993 in six areas: childhood immunizations, breast cancer screenings, cervical cancer screenings, prenatal care, cholesterol screenings and diabetes retinal exams. The 21 HMOs were ranked above average, average or below average for each area.

The California report coincided with the release of a national study on HMO quality and performance produced by the National Committee for Quality Assurance, a nonprofit group that accredits HMOs. Both reports used standard quality measures-known as the Health Plan Employer Data and Information Set, or HEDIS-developed by the national committee. The national study judged selected HMOs on more than 30 measures of quality, patient satisfaction and performance, including how many people quit each year and how long they stayed in the hospital.