The effect of length of membership upon the utilization of ambulatory care services. A comparison of disadvantaged and general membership populations in a prepaid group practice.

The relationships between duration of membership and the use of outpatient medical care services for disadvantaged and general membership groups in a prepaid group practice were analyzed. The effects of duration of membership upon utilization rates during a six-year study period were estimated by retrospective cohort analyses. Rates were computed for successive three-month intervals. There were no significant start-up effects on use for new enrollees in either study population. This finding is in contrast to what has been reported in other studies. New HMOs or existing HMOs with many new members may not require the additional resources previously thought necessary. Average cohort utilization rates were higher for the disadvantaged cohorts, with the exception of young males 0 to 14 years old. We also examined the persistence of utilization patterns over time. Consistently high users and nonusers were found in both populations. In general, these findings suggest that length of time does not affect ambulatory care use. Age, sex, socioeconomic background and health status are more important than duration of membership in accounting for differences in utilization.