Although there are multiple sources of data on the uninsured, the Census Bureau’s Annual Social and Economics Supplement (ASEC) to the Current Population Survey (CPS) is often the main focus of analytic work. Some of the reasons for this are: it is the most widely cited source of data, it has the largest sample size of any major survey with data on the uninsured, it can be used for state-level analysis, and it contains detailed data allowing for analysis of the uninsured by income level. However, one factor that has complicated the analysis of the uninsured is that this supplement to the CPS has changed considerably over time, making longitudinal analysis less reliable.
Without adjusting for these changes, we are able to look only at the most recent survey years which use a consistent methodology. For CY 2003, the CPS estimates that 45.0 million individuals, or 15.6 percent of the U.S. civilian non-institutionalized population, were without health insurance for the entire year. This number is a full year uninsured estimate based on the structure of the CPS questionnaire – which asks respondents about their insurance coverage at any time during the prior calendar year. When compared, however, to other surveys that examine both full year and point in time uninsured, the CPS estimate appears to more closely resemble the point in time estimate from these other surveys.
While some analysts have suggested, as far back as 1986, that the CPS represents a measure of the uninsured at a point in time, ASPE believes that it is more likely that the CPS does represent the full year uninsured, but that the estimate is inflated due to poor reporting of Medicaid coverage and perhaps other coverage types as well.
In order to more fully examine this premise, it becomes necessary first to adjust for the survey changes that have occurred in the CPS over time. Focusing mainly on the period from CY 1994 (March 1995) forward but looking back as far as CY 1987 (March 1988), these changes (and our adjustments) include: a) updating the survey weights to reflect the new decennial Census, b) adjusting for consistency in the insurance questionnaire which was modified beginning in March of 1995, c) adjusting for consistency in employer sponsored insurance (age of policy-holder, coverage from outside of household), and d) adjusting for inclusion of questions to verify uninsurance and coverage under SCHIP.
Once these changes have been taken into account and adjusted for, the Medicaid undercount can then be considered. For CY 1995, the CPS showed just over 30 million persons covered by Medicaid. CMS data, however, suggests that approximately 39 million non-institutionalized persons were covered under Medicaid some time in that year. By CY 2002, this discrepancy has doubled, with the CPS finding fewer than 29 million ever covered by Medicaid, while CMS data suggests an “ever enrolled in Medicaid” count on the order of 46 million in the noninstitutionalized population1. The CPS counts are lower by subpopulation both when compared to CMS estimates as well as when compared to data from other surveys.
For CY 2003, correcting for an undercount of 17 million persons lowers the full year uninsured estimate by just over 9 million persons. For CY 2003, with this adjustment, the almost 36 million uninsured (as compared to an unadjusted 45.0 million) is more consistent with the full year uninsured count reported by MEPS of 31.7 million (although for an earlier year).
1Explained in more detail in the body of this paper, CMS presents Medicaid enrollment statistics both from MSIS data as well as summary historical counts and projections of the Medicaid population on their website. Our use of these estimates of the Medicaid population is explained in detail in the Technical Appendix to this document
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