The SIPP asks respondents whether they were covered by employer- or union-sponsored insurance, other private health insurance, Medicare, military health care, or Medicaid. Like the CPS, estimates of the uninsured using the SIPP are calculated as a residual -- that is, the uninsured are those who do not report receiving coverage of any type. Unlike the CPS, though, SIPP respondents are asked about health insurance coverage in each month of the 4-month reference period. The health insurance portion of the 1993 SIPP survey instrument is presented in Appendix B.
The SIPP also suffers from underreporting of Medicaid. For example, HCFA(now known as CMS) administrative data show that 35.7 million persons were ever enrolled in Medicaid in 1992. In comparison, Bennefield (1996c) calculated that 12.3 percent of all persons, or approximately 30.5 million persons, reported Medicaid for at least one month in 1992 based on the SIPP -- an underreporting of about 15 percent, or roughly the same magnitude of the underreporting on the CPS. Therefore, the number of uninsured based on the SIPP may be overestimated somewhat, assuming that private health insurance is reported accurately (or, at least, not overreported). Also like the CPS, the SIPP suffers from undercoverage of the population in general. According to the Census Bureau, though, the final impact of undercoverage on estimates is unknown.
Various SIPP estimates of the uninsured, even those for the same time period, may not be comparable because there are a number of different alternatives for analyzing a given time period based on the SIPP. Some examples of these alternatives are as follows:
- Because SIPP panels overlap, researchers often have a choice of SIPP panels for a given time period, or researchers can combine SIPP panels.
- The weights researchers use will depend on the length of the time period analyzed and the specific SIPP file used. Researchers may use calendar year weights, panel weights, or wave-specific weights.
- Some researchers may have used sophisticated duration estimates, while others may have used simple slice-in-time analyses.
This last point deserves further explanation. Researchers' estimates of the uninsured using the SIPP can vary substantially depending on whether they use duration estimates for all spells observed over a period of time or whether they simply examine spells in progress at a point in time. Swartz and McBride (1990) pointed out that studies of the dynamics of health insurance coverage that concentrate on uninsured spells already in progress are misleading because spells in progress at a point in time are disproportionately long spells, whereas most spells are actually fairly short. Using data from the 1984 panel of the SIPP, Swartz and McBride demonstrated this phenomenon by comparing the distribution of spell lengths for all persons for whom they could observe a spell beginning in the SIPP with the distribution of spell lengths among persons whose spells were in progress at a point in time. Using a survival analysis technique, they found that half of all observable spells ended within 5 months, and another 16.5 percent ended within 5 to 8 months. Only 15 percent of all spells lasted more than 2 years. In contrast, among spells in progress at a point in time, 58 percent lasted more than 2 years and only 13 percent ended within 5 months.
Although researchers have calculated estimates for a wide variety of time periods, this section discusses only a few estimates simply to give an overall picture of the uninsured according to the SIPP. In addition, we do not attempt to compare and contrast estimates because, as stated above, estimates can vary based on the SIPP files and methodology used and most researchers do not publish their precise methodology. A more detailed set of estimates that researchers have produced is presented in Tables II.3 (for children) and II.4 (for all persons).(14) First, though, we compare SIPP estimates of the uninsured with the corresponding CPS estimates.