Recent census data indicate that, with the exception of two counties, the out migration rate is just under four percent every four years. This stable population provides unique opportunities for longitudinal research. Geisinger's service region has a higher proportion of elderly, both over 70 and over 85, than the United States population. Pennsylvania possesses the third highest proportion of elderly among the 50 states, with currently 17.4 percent of the population being over 65, and possesses one of the largest population of rural elders. The primary care population of Geisinger Clinic is similar to the region's resident population, except the proportion of males is lower (i.e., 45 percent vs. 55 percent), primarily due to relatively fewer young adult males, compared to females.