Using National Survey Data to Analyze Children’s Health Insurance Coverage: An Assessment of Issues. 1. The Major Surveys

05/21/1999

The CPS is a monthly survey whose chief purpose is to provide official estimates of unemployment and other labor force data. In an annual supplement administered each March, the CPS captures information on the health insurance coverage. In large part because of the timely release of these data and their consistent measurement over time, the CPS has become the most widely cited source of information on the uninsured. The March supplement is also the source of the official estimates of poverty in the United States. The availability of the poverty measures along with the data on health insurance coverage and a large sample size--50,000 households--that can support state-level estimates have contributed to making the CPS an important resource for research on the uninsured.

The National Health Interview Survey (NHIS) collects data each week on the health status and related characteristics of the population. The principal purpose of the NHIS is to provide estimates of the incidence and prevalence of both acute and chronic morbidity. To achieve this objective, the entire year must be covered. To limit the impact of recall error and reduce respondent burden, the annual interviews (with more than 40,000 households) are distributed over 52 weeks, and respondents are asked to report on their current health status as well as recent utilization of health care services. The interviews include a battery of questions on health insurance coverage. These data can be aggregated over the year to produce an average weekly measure of insurance coverage. Despite some clear advantages of the NHIS measure over the CPS measure of the uninsured, however, the NHIS measure has been much less widely accepted and cited. Even its limitations are much less well known than those of the CPS measure. The long lag with which data from the NHIS are released, relative to the March CPS, is undoubtedly a major factor limiting use of these data on uninsurance.

The last of the three ongoing surveys, the SIPP, is a longitudinal survey that follows a sample of households--a “panel”--for two-and-a-half to four years. Sample households are interviewed every four months and asked to provide detailed monthly data on household composition, employment and income of household members, and other characteristics. Each interview includes a battery of questions on health insurance coverage. Until a major redesign, initiated in 1996, new panels were started every year. When combined, the overlapping panels yielded national samples that were about three-quarters the size of the CPS and NHIS samples. The 1996 panel, which is twice the size of its predecessors, will run for four years; the next panel is not scheduled to begin until 2000. While the enhanced sample size was intended to eliminate the need for overlapping panels, starting a new panel every year also provided a way to maintain the representativeness of SIPP data over time. The loss of overlapping panels, however, weakens the SIPP as a source of reliable data on national trends. Finally, while the redesign has also slowed the release of data from the 1996 panel, SIPP data have never been released in as timely a manner as March CPS data, and, as with the NHIS, this has limited their value as a source of current data on trends.(1)

All three of these surveys are conducted by the U.S. Bureau of the Census. The CPS is a collaborative effort with the Bureau of Labor Statistics (BLS), which bears ultimate responsibility for the labor force statistics. The March supplement and the SIPP, however, are entirely Census Bureau efforts. The NHIS is conducted for the National Center for Health Statistics (NCHS), with the Census Bureau serving, essentially, as the survey contractor.

Periodically, the Agency for Health Care Policy and Research (AHCPR) conducts a panel survey of households to collect detailed longitudinal data on the population’s utilization of the health care system, expenditures on medical care, and health status. The most recent of these efforts, the Medical Expenditure Panel Survey (MEPS), was drawn from households that responded to the NHIS during the middle quarters of 1995. The initial MEPS interviews were conducted by Westat. Like the SIPP, MEPS will collect data at subannual intervals, and new panels will overlap earlier panels, allowing data to be pooled to enhance sample size and improve representativeness (see Section E).

The federal government is not alone in sponsoring large-scale national surveys to measure health insurance coverage and aspects of health care utilization. Private foundations have sponsored a number of surveys as well. While none of these foundation-sponsored efforts has been repeated with sufficient regularity to provide a long-term source of data on trends, the two most prominent of the recent undertakings will collect data from at least two points in time. The household component of the Community Tracking Study (CTS) was conducted by Mathematica Policy Research for the Center for Studying Health System Change, with funding from the Robert Wood Johnson Foundation.(2) The survey was fielded between July 1996 and July 1997 and collected data on current health insurance coverage (that is, at the time of the interview). Interviews were completed with about 32,000 families representing the civilian noninstitutionalized population of the 48 contiguous states and the District of Columbia. More than a third of the sample was concentrated in 12 urban sites that will be the subject of intensive study. The second round survey, which includes both a longitudinal component and a new, independent sample of households, started in 1998 and will be completed in 1999.

In 1997 the Urban Institute, with sponsorship from a group of foundations, fielded the first wave of the National Survey of America’s Families (NSAF).(3) The total sample size of 44,000 households compares to the NHIS, although the nationally representative sample (except for Alaska and Hawaii) features large samples for 13 states. These 13 states, which account for one-half of the U.S. population, will be the subject of intensive study. The survey was conducted by Westat from February through November of 1997. A second interview with the same sample is currently in the field, and a third interview may be fielded as well. Both the CTS and the NSAF include extensive batteries of questions on health insurance coverage, and both incorporate significant methodological innovations in these measures, which we will describe shortly.

Table 1 presents estimates from each of these surveys of the proportion of children who were uninsured at different times between 1993 and 1997. With the exception of the MEPS estimate, discussed below, all of these estimates represent or are widely interpreted to represent children who were uninsured at a point in time. Estimates refer to children under 19 (CPS and SIPP) or children under 18.(4) We will refer back to this table as we discuss alternative approaches to measuring uninsurance and the sources of error in estimates of the uninsured. Briefly, however, the estimates from the CPS, which we have reported for all five years, show little movement over the first three years but then a 1.1 percentage point rise between 1995 and 1996, with essentially no change between 1996 and 1997. The NHIS estimate in 1993 equals the CPS estimate, but the NHIS series shows a 1.2 percentage point rise between 1993 and 1994, followed by a 1.7 percentage point drop

                                   TABLE 1
ESTIMATES OF THE PERCENTAGE OF CHILDREN WITHOUT HEALTH INSURANCE, 1993-1997
                                                                                          
Source of     1993        1994        1995        1996       1997
Estimate
                                                                                          
CPS           14.1        14.4        14.0        15.1       15.2
NHIS          14.1        15.3        13.6        13.4        --
SIPP          13.9        13.3         --          --         --
MEPS           --           --          --        15.4        --
CTS            --           --          --        11.7        --
NSAF           --           --          --         --         11.9
                                                                                          
Notes:  Estimates from the CPS and SIPP are based on tabulations of public use
files by Mathematica Policy Research, Inc., and refer to children under 19 years
of age.  Estimates from the other surveys apply to children under 18.  The NHIS 
estimates were reported in NCHS (1998).  The estimate from MEPS refers to children
who were "uninsured throughout the first half of 1996," meaning three to six 
months depending on the interview date; the estimate was reported in 
Weigers et al. (1998).  The CTS estimate, reported in Rosenbach and Lewis (1998), 
is based on interviews conducted between July 1996 and July 1997.  The NASF estimate, 
reported in Brennan et al. (1999), is based on interviews conducted between 
February and November, 1997.
 

between 1994 and 1995 and then essentially no change between 1995 and 1996, at which point the NHIS estimate is 1.7 percentage points below the CPS estimate. We should caution, however, that the 1996 NHIS estimate is a preliminary figure based on just the first 5/8 of the sample. For this reason it may not reflect the impact of the implementation of the Personal Responsibility and Work Opportunity Reconciliation Act (PRWORA)--the welfare reform law that went into effect in the late summer of 1996. Some observers have attributed the rise in the CPS estimate of uninsured children between 1995 and 1996 to a reduction in the Medicaid caseload that accompanied the implementation of welfare reform (Fronstin 1997). The SIPP estimate for September 1993, at 13.9 percent, lies within sampling error of the CPS and NHIS estimates for 1993, but the SIPP estimate drops between 1993 and 1994 while both the other series rise. Like the CPS estimate, the MEPS estimate of 15.4 percent purports to be children who were continuously uninsured over a period of time (three to six months in this case), but its value, which nearly equals the CPS estimate, is more consistent with point-in-time estimates. Finally, both the CTS and the NSAF yield estimates below 12 percent for the proportion of children who were uninsured. These estimates for the privately funded surveys lie substantially below the estimates from the federal surveys. In later sections we will explore possible reasons for this difference.