Analysis of Children's Health Insurance Patterns: Findings from the SIPP. Representativeness of the SIPP Sample

05/12/1999

At the time of the initial interview, the children in the SIPP sample represented the entire United States population of children except for those residing in institutions or military barracks.1 Over time, however, as the U.S. population of children under 19 grew to 74.0 million, the population of children represented by the SIPP longitudinal sample declined to less than 70 million. As we detailed in Technical Appendix A, except for births the SIPP sample does not represent children who entered the population after the initial interview--that is, children who migrated to the U.S., returned from abroad, or left institutions. Moreover, while the SIPP sample theoretically represents all children born to members of the initial population who remained in the U.S., we demonstrated that these births are underrepresented by as much as a quarter. It is this shortfall of sample births that accounts for the sizeable decline in the population of children that the SIPP sample represents.

The fact that most of the divergence between the SIPP population and the total U.S. population of children is concentrated in the youngest ages carries important implications. As we have seen, the health insurance coverage patterns of very young children are strikingly different from those of other children. Underrepresenting births will very likely affect the overall distribution of coverage, increasing the proportion uninsured and reducing the proportion covered by Medicaid

The fact that the SIPP sample does not represent immigration has implications for estimates of insurance coverage as well. We would expect the rates of insurance coverage to be lower among immigrant children than among the general population of children. Therefore, excluding the immigrants will lower both the absolute number and the proportion of children who are estimated to be without insurance relative to what we would observe with a sample of the total population at the same point in time.

In order to evaluate our SIPP estimates of Medicaid enrollees against those reported by the Health Care Financing Administration(now known as Centers for Medicare and Medicaid Services(CMS)) (HCFA(now known as CMS)), we must expand our estimates of Medicaid coverage to the entire population. This entails first estimating the number of children that we are "missing" due to the SIPP panel's underrepresentation of births and its non-representation of other additions to the population and then estimating the frequency of Medicaid coverage and lack of insurance among these additional children. Our derivation of adjusted population totals, estimates of uninsured children, and estimates of children enrolled in Medicaid is summarized in Table 1.

Year and Estimate of Year at End Under 19 Children in Year Uninsured Ever Number of Year at End Uninsured Number in Year Medicaid Enrolled in Number Ever of Year at End Medicaid Enrolled in
TABLE 1: ADDITIONS TO THE PANEL ESTIMATES OF CHILDREN EVER UNINSURED AND CHILDREN EVER ENROLLED IN MEDICAID, FY93 AND FY94
FY93          
Panel Estimate 70,868,000 16,089,000 9,271,000 17,800,000 13,369,000
Additions to Correct for Underrepresentation of Births 1,150,000 169,000 75,000 441,000 382,000
Additions to Include Other New Entrants to the Population 1,119,000 241,000 147,000 266,000 211,000
Adjusted Total 73,137,000 16,499,000 9,493,000 18,507,000 13,962,000
FY94          
Panel Estimate 69,935,000 15,936,000 8,911,000 17,795,000 13,259,000
Additions to Correct for Underrepresentation of Births 2,250,000 360,000 181,000 843,000 716,000
Additions to Include Other New Entrants to the Population 1,772,000 383,000 225,000 429,000 337,000
Adjusted Total 73,957,000 16,679,000 9,317,000 19,067,000 14,312,000
SOURCE: Panel estimates are taken from Technical Appendix A, Tables 1 and 4. Adjusted total populations are also from Table 1. Estimates of the proportion of infants and young children uninsured or enrolled in Medicaid are based on Technical Appendix C, Tables 24 and 27. The estimates of the proportion of children uninsured or enrolled in Medicaid that were applied to new entrants were taken from Technical Appendix A, Table 4.

For September 1993 we added 1.150 million children to correct for the underrepresentation of births and another 1.119 children to account for net growth of the population of children. For September 1994 we added 2.250 million children to correct for the underrepresentation of births and another 1.772 million children to incorporate other new entrants to the population.

To derive estimates of uninsured children and Medicaid enrollees among the children added to correct for underrepresented births, we applied age-specific rates from two tables that were presented in Technical Appendix C. To derive estimates of uninsured children and Medicaid enrollees among the other new entrants to the population, we assumed that these children shared the same probabilities of being uninsured or covered by Medicaid as all children in 1993 and 1994, and we applied rates from a table presented in Technical Appendix A. This is not an entirely satisfactory assumption, given that these new entrants to the population could differ dramatically from the rest of the population. Nevertheless, in the absence of relevant data on this population, the assumption is a reasonable one to apply in this case, and the final results are not very sensitive to it.

The net effect of these operations is to add about 700,000 children to the number of children estimated to have been ever enrolled in Medicaid during FY93 and 1.27 million to the number estimated to have been ever enrolled in Medicaid in FY94. Children added to correct for the underrepresentation of births accounted for about two-thirds of the additional Medicaid enrollees in each year.

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