Our use of the CPS ASEC implicitly assumes that the counties in the survey sample are representative of those not selected. The CPS was designed so that Primary Sampling Units (PSUs) are representative of their strata, primarily for unemployment, but the degree to which the CPS ASEC sample is representative for health insurance coverage is unknown. The characteristics of some counties guarantee they are included, e.g., most counties in large metropolitan areas and counties with large populations. More generally, while all counties have a nonzero probability of being included in the sample, some have higher probabilities than others. Further, the probability of selecting a county is related to its income and poverty level which, in turn, are related to the level of health insurance coverage. In the related Small Area Income and Poverty Estimates (SAIPE) program, comparison of regression equations based on census data for counties in the CPS ASEC sample and equations based on all counties indicates remarkably similar results, providing some assurance that the CPS ASEC counties are largely representative of all counties for poverty. Unfortunately, the analogous test is unavailable for health insurance coverage, since there are no health insurance questions on the decennial census.
The survey weights used in estimation at the national level are not appropriate for county-level estimates. The CPS ASEC sample design selects some PSUs (usually a county or group of counties) to represent a set of counties in the same stratum. The sum of the weights for sample households from such a county estimates the total population of the entire set of counties it represents. Because we want each county in the CPS ASEC sample to stand for itself, we have adjusted the weights to make the direct estimate for each county approximately unbiased.