Percent of full-time medical care participants by plan type, medium and large private establishments, selected years, 1991-97 Percent 1991 1993 1995 1997 Traditional fee-for-service 66 50 37 26 Preferred provider organization 16 26 34 40 Prepaid health maintenance organization 17 23 27 33 Note: The categories "exclusive provider organization" and "not determinable" are not shown on chart.