Health Insurance: Federal Data Sources for Analyses of the Uninsured

Health Insurance: Federal Data Sources for Analyses of the Uninsured

Order Code RL31275
CRS Report for Congress
Received through the CRS Web
Health Insurance: Federal Data Sources
for Analyses of the Uninsured
February 7, 2002
Chris L. Peterson and Christine Devere
Analysts in Social Legislation
Domestic Social Policy Division
Congressional Research Service ˜ The Library of Congress

Health Insurance: Federal Data Sources
for Analyses of the Uninsured
Summary
The number of people without health insurance continues to be a key public
policy concern. Numerous sources of data, both public and private, gather
information on the uninsured but they produce estimates that vary widely. This report
discusses estimates of the number of uninsured using data from four federally
administered surveys: the March Supplement to the Current Population Survey
(CPS), the Survey of Income and Program Participation (SIPP), the National Health
Interview Survey (NHIS), and the Medical Expenditure Panel Survey (MEPS).
In 1998 (the most recent year for which data are available from all four surveys),
estimates of the number of uninsured for the full year ranged from 21.2 million using
SIPP data to 43.6 million using data from the March CPS, a difference of more than
22 million individuals. Estimates of the number of uninsured at a point in time as well
as the number who were ever uninsured during the year also vary among MEPS,
NHIS, and SIPP (the March CPS does not support these estimates). The number of
uninsured at a point in time in 1998 ranged from 39.2 million from SIPP to 42.3
million from MEPS. Estimates of the number who were ever uninsured during 1998
ranged from 50.0 million from NHIS to 60.3 million from MEPS.
To date, no clear consensus has emerged that explains why these differences
exist in the survey estimates of the uninsured. However, researchers have offered a
number of explanations that may account for some of the variation. These include:
! the surveys ask different questions and use different methodologies to
determine who is insured;
! compared to administrative data, the surveys report estimates of the percent
enrolled in Medicaid that indicate an under-reporting of Medicaid coverage by
survey respondents, which would affect uninsured estimates;
! the surveys differ in the amount of time for which they ask individuals to
remember their health insurance status. Individuals who are asked to report
their status for a period of time might not accurately report changes in their
health insurance status over that period; and
! the surveys are designed in ways that permit different types of analyses, but
may limit comparability of the results. For example, while the SIPP and MEPS
allow researchers to track the same individuals over a period of years, this is
not possible using the CPS or NHIS. Therefore, the varying designs of the
surveys permit different types of analyses of health insurance status, but the
comparability of the estimates may be limited.
The March CPS produces the most widely cited uninsured statistics, as the data
are timely and can produce estimates for each of the 50 states and the District of
Columbia. While differences in the uninsured estimates persist among the surveys, the
NHIS, SIPP, and MEPS offer additional data to explore questions about the
uninsured. For example, the NHIS contains data to relate health insurance status to
health risk factors and general health issues, while SIPP and MEPS both are useful in
examining changes in health insurance and other characteristics over time.

Contents
Overview of the Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
March Supplement to the Current Population Survey (CPS) . . . . . . . . 3
Survey of Income and Program Participation (SIPP) . . . . . . . . . . . . . 3
National Health Interview Survey (NHIS) . . . . . . . . . . . . . . . . . . . . . 4
Medical Expenditure Panel Survey (MEPS) . . . . . . . . . . . . . . . . . . . . 4
Comparing Uninsured Statistics Among the Surveys . . . . . . . . . . . . . . . . . 5
Potential Reasons for Variation in Survey Estimates of the Uninsured . . . . 7
Health Insurance Verification Question . . . . . . . . . . . . . . . . . . . . . . . 7
Medicaid Under-reporting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Recall Period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Longitudinal Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Full-year Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
List of Figures
Figure 1. Uninsured U.S. Civilian Noninstitutionalized Population from
Federally Administered Surveys, 1998 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 2. Source of Health Insurance Coverage for Any Part of 1999,
March CPS and SIPP, Percent of U.S. Civilian Noninstitutionalized
Population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 3. Ever Covered by Medicaid During 1998,
U.S. Civilian
Noninstitutionalized Population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
List of Tables
Appendix A. Comparison of Characteristics of Federally Administered
Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Appendix B. Uninsured U.S. Civilian Noninstitutionalized Population,
from Federally Administered Surveys, 1998 . . . . . . . . . . . . . . . . . . . . . . . 19

Health Insurance: Federal Data Sources
for Analyses of the Uninsured
The number of individuals without health insurance continues to be a key public
policy concern. The majority of the U.S. population receives coverage through
employment-based plans, and employers generally contribute to the cost of purchasing
health insurance. Changes in employment often lead to a disruption or loss of health
coverage, as individuals who lose an employer’s health benefit contribution often
consider the cost of purchasing health insurance on their own too high. Other
working people do not have insurance because their employer does not offer health
insurance coverage. For these individuals, policies to reduce the cost of health
insurance (such as tax credits or other types of subsidies) may increase their
enrollment in health coverage. Public programs such as Medicaid and the State
Children’s Health Insurance Program (SCHIP) provide health insurance for certain
groups of low-income families with children. But, these programs have eligibility
requirements that may prevent other uninsured individuals from qualifying for
coverage. At the same time, a number of individuals eligible for health insurance
through these public programs are not enrolled and are therefore included in current
estimates of the number of uninsured. For these groups, additional outreach activities
could increase enrollment in these existing programs.
Given the various reasons individuals lack health insurance coverage, analysis of
the effectiveness of a proposal to reduce the number of uninsured (such as those
discussed above) requires examining characteristics of individuals with and without
health insurance coverage. Numerous sources of data have emerged over time that
support estimates of the uninsured. However, each data source differs in how it
collects information from individuals, as well as the amount of information it collects
related to health insurance status. Therefore, the estimates of the number of
uninsured produced by these data sources vary widely.1 Because proposals to reduce
the number of uninsured are informed by estimates of the number of individuals
without health insurance as well as the characteristics of the uninsured population,
determining the appropriate source of data to evaluate these proposals is important.
1 A number of studies have attempted to explain why the data sources differ in their estimate
of the uninsured. See, for example, U.S. Department of Health and Human Services (DHHS)
Office of the Assistant Secretary for Planning and Evaluation. Understanding Estimates of
the Uninsured: Putting the Differences in Context.
[http://aspe.os.dhhs.gov/health/reports/hiestimates.htm]; Lewis, Kimball, Ellwood, Marilyn
and Czajka, John L. Counting the Uninsured: A Review of the Literature. The Urban
Institute, June 1998, [http://newfederalism.urban.org/html/occ8.htm]; and Fronstin, Paul
Counting the Uninsured: A Comparison of National Surveys by Employee Benefit Research
Institute, Issue Brief No. 225, September 2000.

CRS-2
This report outlines the major advantages and limitations of four federally
administered surveys2 — the March Supplement to the Current Population Survey
(CPS), the Survey of Income and Program Participation (SIPP), the National Health
Interview Survey (NHIS), and the Medical Expenditure Panel Survey (MEPS) — in
providing estimates of the uninsured population.3 It begins by briefly describing the
characteristics of each of these four surveys. Estimates of the uninsured from these
datasets are then presented, followed by a discussion of possible reasons for the
variation among the estimates. Finally, the report discusses potential areas in which
each data source might be useful to respond to questions of interest to policymakers
in outlining proposals to reduce the number of uninsured.
In brief, all four surveys support representative estimates of the uninsured U.S.
civilian noninstitutionalized population, although the health insurance questions asked
among the surveys differ. The March CPS produces the most widely cited statistics
on the number of uninsured, because the data provide the most recent full-year
information and because the data may produce estimates that are representative for
each of the 50 states and the District of Columbia. However, its focus is not health
insurance but work, income and poverty statistics, and it does not delve deeply into
issues of insurance coverage. Additionally, the March CPS provides estimates only
of those who were without health insurance for the entire year as opposed to those
without insurance for some portion of the year. The NHIS, SIPP, and MEPS can not
provide state-level analysis similar to the March CPS, but these surveys do offer
additional data to explore many questions about the uninsured. For example, the
NHIS contains data to relate health insurance status to health risk factors and general
health issues. Additionally, SIPP and MEPS collect data for every month that the
individual is part of the survey and therefore, in addition to providing estimates of
those without health insurance for an entire year, these surveys may also be used to
examine changes in health insurance and other characteristics month-to-month or over
a period of time, tasks that are not possible using the March CPS. MEPS also asks
the most comprehensive questions about the type and source of people’s health
insurance. With data from these four surveys, it is possible to answer many questions
about the uninsured. Because the estimates among the surveys differ by so much,
however, the results should always be interpreted with some caution until the
differences can be reconciled.
2 There are other federal sources for estimates of the uninsured, including the Behavioral Risk
Factor Surveillance System (BRFSS) and the National Survey of Family Growth (NSFG).
We did not include these particular surveys because they are not representative of the entire
U.S. population. BRFSS collects information only on the adult who answers the phone, which
excludes all of the children and the other adults in the household. The responding adult is not
necessarily representative of the rest of the household, particularly the children. NSFG, a
valuable survey of women’s health, limits its sample to women aged 15 to 44. In addition, it
does not occur annually; the latest survey was conducted in 1995. There are also non-federal
sources for estimates of the uninsured. Analysis of these sources is beyond the scope of this
report.
3 We chose to discuss the surveys in this order simply for the benefit of our readers. It is not
intended to represent any survey’s importance or reliability.

CRS-3
Overview of the Surveys
Appendix A illustrates in a tabular format the characteristics of the four surveys
discussed in this report. Each of these surveys is briefly described below.
March Supplement to the Current Population Survey (CPS). The
CPS, conducted by the U.S. Census Bureau, is the primary source of information on
the labor force characteristics of the U.S. civilian noninstitutionalized population. The
CPS is a cross-sectional survey, which means that it does not have the ability to track
the same individuals from year to year. Although CPS interviews take place every
month, the questions regarding health insurance status are asked only in March, in the
CPS’s Annual Demographic Survey (often referred to as “the March supplement”),
at which time individuals are asked to identify their sources of health insurance for the
entire preceding year. Therefore, the March supplement to the CPS (herein referred
to as the March CPS) supports estimates only of those who do not have health
insurance for the entire year preceding their March interview. Of the four surveys
reviewed in this report, the CPS has the largest sample size (approximately 50,000
households) and is the only one of the four surveys able to produce representative
estimates for each of the 50 states and the District of Columbia. The formula that
allocates funds to states for the SCHIP uses CPS state-level data.4
The Census Bureau releases data from the March CPS around September every
year. The most recent data available are from the March 2001 CPS, and these data
represent the previous calendar year (so the March 2001 CPS represents information
for all of 2000). Of the four surveys reviewed in this report, the March CPS data
provide the most recent full-year information on the uninsured. However, because
the March CPS data is not designed to allow researchers to track individuals who may
remain in the sample from year to year, it cannot pinpoint changes in an individual’s
health insurance status across time. Therefore, researchers using the March CPS can
estimate the total number of uninsured for a given year and compare the total number
of uninsured across a number of years (for example, 1997 through 1999), but can not
examine individual changes in health insurance coverage throughout the year, or
estimate the number of uninsured at a specific point in time during the year.
Survey of Income and Program Participation (SIPP). In addition to the
CPS, the Census Bureau also conducts the SIPP, a survey with detailed information
on respondents’ income as well as their participation in government programs. Unlike
the CPS, the SIPP is a longitudinal survey, which means that individuals who are part
of the sample are interviewed multiple times over the life of the survey, referred to as
a “panel,” so that changes in various individual characteristics may be observed.
Respondents in the SIPP sample are interviewed every four months, with these cycles
referred to as “waves.” Historically, the SIPP panels ranged in duration from 2½
4 For each state, the allocation is based on a 3-year average of the number of uninsured
children under 19 years of age in families below 200% of the poverty thresholds. In some
states, the number of children in these low-income families for whom the CPS obtains data
may be small. This may cause the estimates to be less reliable and increase the variability in
the estimates of the uninsured from year to year. The 3-year averages are used to counter
these effects.

CRS-4
years to 4 years. Although representative at the national level, SIPP does not support
reliable state-by-state estimates. The sample size is approximately 37,000 households.
The most recent available data are from the 1996 SIPP panel, which followed a
sample of individuals for 4 years, through March 2000.5 Given the longitudinal design
of the SIPP (as discussed above), these data may be used to estimate the total number
of people uninsured for any month, for any part of the year, for an entire year, or over
the life of the panel. The next SIPP panel, which will be over a 3-year period, began
earlier this year, with the first wave of results expected in the first quarter of 2002.
However, there is no set schedule for SIPP data releases.
National Health Interview Survey (NHIS). The National Center for Health
Statistics (NCHS) oversees NHIS, a survey completed annually to gather information
on Americans’ health. Similar to the CPS, the NHIS is a cross-sectional survey and
therefore does not allow analyses of the same people from year to year. NHIS is
representative of the U.S. civilian noninstitutionalized population and has the ability
to produce estimates for some large states; however, state-level estimates for all 50
states and the District of Columbia are not supported. The sample size is
approximately 42,000 households.
The most recent available data from NHIS are for 2000. Although NHIS is
similar to the CPS because it is a cross-sectional survey, the NHIS collects more
detailed information on health insurance status among the survey sample than the
March CPS. Therefore, NHIS can produce estimates of the total number of people
who were ever uninsured during the year, for the entire year, or at the time of the
survey. In addition to health insurance status, NHIS collects detailed health
information on illnesses, chronic conditions, activity limitations and health risk
behaviors. Another benefit of NHIS is that the survey serves as the sampling
framework for a number of other smaller, federally administered surveys such as the
Medical Expenditure Panel Survey (discussed below) and the National Survey of
Family Growth (NSFG). Therefore, individuals in these smaller surveys have also
been interviewed in NHIS. By linking these datasets, researchers may have a broader
collection of health-related information for a subset of the population.
Medical Expenditure Panel Survey (MEPS). The Agency for Healthcare
Research and Quality (AHRQ) administers MEPS in conjunction with NCHS. MEPS
has been conducted annually since 1996. It is the most recent in a series of medical
expenditure surveys that began in 1977 as the National Medical Care Expenditure
Survey and later became the National Medical Expenditure Survey (NMES). The
primary focus of MEPS is on the use of and expenditures for health care. MEPS is
a longitudinal survey, like the SIPP, and interviews the same individuals five times
over a period of 2 years. MEPS does not support state-level estimates of the
uninsured, although (like the other three surveys discussed above) it does provide a
nationally representative picture of the U.S. civilian noninstitutionalized population.
5 The core content of these files is available. The SIPP also includes additional survey
questions that are included in selected waves on special topics. These questions are referred
to as “topical modules.” Examples include topical modules on health care expenditures and
utilization, disabilities and limitations, and home health care.

CRS-5
MEPS has the smallest sample size of the surveys, with 12,000 eligible households
interviewed each year.
Unique to MEPS are the “overlapping panels” — although individuals are
followed for 2 years, each year a new sample is drawn from the previous year’s NHIS,
so the sample is replenished annually.6 For example, the 1998 full-year MEPS data
represents information from year 2 of households in the 1997 panel and year 1 of
households in the 1998 panel. The implications of this are discussed in greater detail
later in this report.
The most recent available full-year data for MEPS is 1998. However, MEPS
also produces “point-in-time” files, which represent the first 6 months of a given year.
Point-in-time information is available for 2000. Researchers using the full-year MEPS
data can estimate the total number of uninsured for any month, for any part of the
year, for an entire year, or over the life of the panel. In addition to health insurance
status, MEPS supplies the most detailed information about Americans’ health
insurance benefits and premiums, as well as on health care use and spending.
Comparing Uninsured Statistics Among the Surveys
Figure 1 illustrates statistics on the uninsured from 1998, the most recent year
in which the full data from all four surveys are available. All four surveys provide a
nationally representative picture of the U.S. population, however the estimates of the
uninsured vary widely among the surveys, as Figure 1 illustrates. These uninsured
estimates are presented in a tabular format in Appendix B.
In 1998, using data from the March CPS, 43.6 million individuals (16.3% of the
U.S. civilian noninstitutionalized population) were uninsured for the entire year. This
compares to 31.5 million (11.8%) from MEPS. NHIS estimates that 25.8 million
(9.7%) were uninsured for all of 1998. Using SIPP data, this number was 21.2
million (7.9%), which is less than half of the March CPS estimate — a difference of
more than 22 million people.7
6 As mentioned above, the NHIS serves as the sampling framework for MEPS, and therefore
the MEPS survey sample is a subsample of the previous year’s NHIS sample (which is
considerably larger).
7 For SIPP and MEPS, the percentage of uninsured for the full year excludes those who did
not provide data for all 12 months.

CRS-6
Figure 1. Uninsured U.S. Civilian Noninstitutionalized Population
from Federally Administered Surveys, 1998
(in millions of individuals)
70
60
MEPS
Range of
SIPP
Estimates:
10.3 million
50
NHIS
March CPS
MEPS
Range of Estimates:
40
NHIS
3.1 million
Range of Estimates: SIPP
MEPS
22.4 million
30
NHIS
SIPP
20
10
0
Uninsured for a full year
Uninsured at time of survey
Ever uninsured during the year
Source: Congressional Research Service (CRS) analysis of the March CPS, SIPP, NHIS, and MEPS.
Notes: As discussed in the memorandum, the March CPS does not support estimates of the number
uninsured at the time of the survey or the number ever uninsured during the year, and therefore is
not illustrated. Estimates for the number uninsured at the time of the survey (a point-in-time
estimate) using SIPP data are from March 1998, while NHIS data are from interviews throughout
the year and MEPS data are from the first half of 1998. For SIPP and MEPS, the percentage of
uninsured for the full year and at any time excludes those who did not provide data for all 12 months.
Examining the number uninsured at a point in time yields higher estimates than
the full-year analysis as illustrated in Figure 1: 39.2 million (14.6%) from SIPP, 39.5
million (14.7%) from NHIS, and 42.3 million (15.8%) from MEPS (the March CPS
does not support these estimates).8 Compared to the number uninsured for an entire
year, the point-in-time estimates among these three surveys is higher because the
number of people uninsured at a point in time would include those uninsured for the
entire year plus those individuals who were uninsured at the time of the survey.
8 For the point-in-time estimate, SIPP data are from March 1998, NHIS data are from
interviews throughout the year, and MEPS data are from the first half of 1998.

CRS-7
Finally, Figure 1 also illustrates estimates of those ever uninsured during the
year from NHIS, MEPS, and SIPP (the March CPS does not support these estimates).
As illustrated in Figure 1, these estimates are higher than the point-in-time estimates
produced in each survey. This is expected, since the estimate of those who were ever
uninsured during the year would include everyone uninsured at the time of the survey
plus those individuals who were uninsured at any other time during the year. NHIS
estimates that 50.0 million individuals (18.9%) were ever uninsured in 1998. This
compares to 55.2 million (20.7%) using SIPP and 60.3 million (22.7%) based on
MEPS.9
Potential Reasons for Variation in Survey Estimates of the
Uninsured
As Figure 1 illustrates, the estimates of the uninsured among the surveys vary
substantially. The March CPS produces the largest estimate of the number of
uninsured for a full year and SIPP produces the lowest estimate. MEPS produces
estimates of the uninsured that are higher than NHIS and SIPP in every time frame
measured. The following discussion describes differences among the surveys that may
account for some of the variation in the estimates.
Health Insurance Verification Question. One explanation for the
differences in the uninsured estimates may be the variation in how the surveys
determine who is uninsured. Until 2000, the March CPS asked respondents (generally
an adult giving information on everyone in the household) whether anyone in the
family was enrolled in a number of health insurance coverage categories, including
private health insurance, Medicare and Medicaid at any time during the entire
preceding year. If a respondent answered “yes” to any of the sources of coverage, the
name(s) of those with that coverage was requested. Therefore, the March CPS
produces estimates of the number of individuals ever covered by Medicare, ever
covered by Medicaid, etc., in the entire preceding year. The number who do not have
health insurance is calculated by looking at the household roster (given by the
respondent at the start of the interview) to determine if a source of health insurance
coverage was mentioned for every individual. Those in the household whose names
were not given for any source of coverage were then considered uninsured for the
entire preceding year; however, this was not verified with the respondent, as there
was no survey question on the March CPS to confirm that those individuals were
indeed uninsured for the entire preceding year. It is possible that individuals often
may not recognize their family member’s coverage from the interviewer’s list of health
insurance types, or that the respondent may simply neglect to mention a family
member’s name. However, when asked directly to confirm that a family member was
uninsured, respondents may be prompted to share that person’s coverage.
To respond to this concern, a health insurance verification question was added
in the March 2000 CPS to verify the uninsured status of individuals during 1999 (the
entire preceding year). That is, beginning in March 2000, if an individual in the
household was not mentioned as being covered by a health insurance category, the
9 For SIPP and MEPS, as with the full year estimates, the percentage of uninsured at any time
excludes those who did not provide data for all 12 months.

CRS-8
respondent was asked to verify that this individual was in fact uninsured in the entire
preceding year. In an analysis of this new question conducted by the Census Bureau
released in August 2001, about 8% of those previously calculated as uninsured
because they did not indicate that they were covered by any source of health insurance
coverage, when asked directly whether they were in fact uninsured, indicated that they
did have coverage. Because this analysis was conducted after the March 2000 CPS
data had been released, it was not included the original data source. However, the
analysis completed by the Census Bureau indicates that, had these results been
available and included when the data were first released, the Census Bureau would
have reported that the number of uninsured was 39.3 million in 1999 — 3.3 million
less than the 42.6 million previously published. Because of these results, the Census
Bureau is making the verification question a permanent part of the survey and,
beginning with the March 2001 CPS, their estimates of the uninsured will be based
on the verification question.10 However, results without the verification question will
also be available for those wanting to make historical comparisons.11
Figure 2 compares selected health insurance coverage estimates from the March
CPS with and without the verification question to the SIPP, the only other survey that
has released comparable full-year data for 1999. Although the verification question
led to a significant decrease in the number of uninsured reported by the March CPS,
to 39.3 million people (14.3%), it is still much higher than the SIPP estimate of 21.2
million uninsured individuals (7.7%) in 1999 — a difference of 18.1 million people
(6.6 percentage points).
Before the addition of the verification question, Census Bureau staff had
analyzed differences between the March CPS and the SIPP.12 They concluded that
the differences were probably due largely to issues with the March CPS — in
particular, underreporting of private coverage. The addition of the verification
question to the March CPS addressed this in part by increasing the March CPS
estimates of the percent ever covered by private health insurance during the year to
72.1%, compared to the previous estimate of 71.0%. While this narrowed the gap
between the March CPS and the SIPP estimates of the percent enrolled in private
health insurance, a difference of 10.4 percentage points (28.1 million people) remains,
as shown in Figure 2. The verification question also did little to substantially narrow
the gaps between the surveys’ estimates of those ever covered by Medicare or
Medicaid during the year.
10 For information on the insured and uninsured population in 2000 based on the March 2001
CPS, see CRS Report 96-891, Health Insurance Coverage: Characteristics of the Insured
and Uninsured Populations in 2000, by Chris L. Peterson.
11 See Nelson, Charles T., and Robert J. Mills, The March CPS Health Insurance
Verification Question and Its Effect on Estimates of the Uninsured. August 2001,
[http://www.census.gov/hhes/hlthins/verif.html].
12 U. S. Bureau of the Census A Comparative Analysis of Health Insurance Coverage
Estimates: Data from CPS and SIPP, by Robert L. Bennefield, 1996 (Hereafter cited as
Bennefield, A Comparative Analysis of Health Insurance Coverage Estimates).

CRS-9
Figure 2. Source of Health Insurance Coverage for Any Part of 1999,
March CPS and SIPP, Percent of U.S. Civilian
Noninstitutionalized Population
100%
90%
82.5%
80%
71.0% 72.1%
70%
60%
50%
40%
30%
20%
15.5% 14.3%
13.2% 13.2% 13.7%
13.4%
10.2% 10.3%
7.7%
10%
0%
Uninsured
Private
Medicare
Medicaid
March CPS, unverified
March CPS, verified
SIPP
Source: Congressional Research Service (CRS) using census-reported percentages for the March
CPS, and CRS analysis of SIPP.
NHIS is the only other survey reviewed in this report that includes a direct health
insurance verification question.13 Unlike the March CPS data, however, the impact
of the verification question on NHIS estimates of the uninsured has not been
significant.14 This may be because of the detailed followup questions asked of NHIS
respondents when they do not indicate coverage — questions such as “About how
long has it been since you last had health care coverage?” If a respondent had
neglected to mention any source of coverage, these questions may make the
respondent aware of the error and provide the opportunity to correct it. The March
CPS is the only one of the four surveys reviewed in this report that does not ask such
detailed followup questions about respondents’ uninsured status. This could explain
why the verification question yielded a significant difference in estimates of the
uninsured in the March CPS but not NHIS. In fact, staff at AHRQ believe that the
addition of a direct health insurance verification question in MEPS would be
unnecessary because of the detailed followup questions asked in that survey.
13 This direct health insurance verification question was added to NHIS in 1997.
14 From conversations with Robin Cohen, statistician at the National Center for Health
Statistics, Hyattsville, Maryland, who works with the health insurance variables in NHIS.

CRS-10
While the March CPS now includes a question to verify that an individual is
uninsured, it does not attempt to verify the coverage of those who indicate they are
insured. In the 2000 NHIS, such a question was added that directly asks for a
confirmation that the person was covered by comprehensive health insurance.
According to a preliminary analysis by NCHS, most of the respondents who had
originally claimed coverage but changed their answer did so because they were
covered by less-than-comprehensive coverage, such as dental or workers’
compensation insurance. The impact of this verification question was relatively small
and approximately offset the effect of the uninsured-verification question on the
NHIS.15 Unfortunately, it is not possible to know what effect adding a verification
question for those claiming insurance might have on the March CPS estimates.
Medicaid Under-reporting. Another commonly cited concern with the
March CPS estimates of insurance status is the extent to which the number of people
on Medicaid may be underestimated. In 1998, the Centers for Medicare and Medicaid
Services (CMS, formerly known as the Health Care Financing Administration)
reported that 38.6 million noninstitutionalized Americans (14.2%) were covered by
Medicaid. Figure 3 compares CMS’s count of those covered by Medicaid to
estimates from the March CPS, SIPP and MEPS in 1998 (NHIS cannot be used to
estimate the number of people ever covered by Medicaid during the year).
The March CPS data yield the lowest estimate of the number ever covered by
Medicaid in 1998 — 26.4 million people (9.7%), 12.2 million people less than the
number using CMS data.16 As seen in Figure 3, SIPP and MEPS data also produce
estimates of Medicaid coverage that are lower than CMS’s tabulations, though not
as low as the March CPS. Survey respondents may under-report their coverage by
Medicaid because they do not want to admit being covered by a public assistance
program, and this may also contribute to the lower estimates of Medicaid coverage
among the March CPS, SIPP and MEPS when compared to the CMS administrative
data. In addition, if a survey’s list of Medicaid programs does not include the
particular state’s name (or names) of its Medicaid program, respondents may not
affirm that they are on Medicaid because the name of their state-specific program was
not mentioned. For example, a paper from the Employee Benefit Research Institute
includes an instance where a Medicaid managed care plan in Maryland was not listed
in the March 1999 CPS and may have been responsible for undercounting Medicaid
coverage in that state.17 While the March CPS, SIPP and MEPS report a lower
number ever covered by Medicaid than reported by CMS, it is worth noting that some
analysts believe the data supplied by CMS may be of poor quality due to variations
in state reporting styles, double counting of enrollees, and classification errors.18
15 Ibid.
16 The CMS data are for fiscal year 1998; the survey data are for the calendar year 1998.
17 See: Employee Benefit Research Institute. Counting the Uninsured: A Comparison of
National Surveys, by Paul Fronstin.
18 For a discussion of the reliability of Medicaid program data, see CRS Report RL30733,
Medicaid Expenditures and Enrollees, 1998, by Evelyne Baumrucker and Jean Hearne.

CRS-11
The addition of the health insurance verification question beginning with the
March 2000 CPS was expected to reduce the differences in the estimate of the
number ever covered by Medicaid in a given year between the CMS and the March
CPS. Although this report focuses on 1998 data in examining the underreporting of
Medicaid, the addition of the health insurance verification question to the March 2000
CPS resulted in approximately 331,000 more people being classified as ever covered
by Medicaid in 1999 (again, this was the first year this verification question was
included as part of the survey). Therefore, while the addition of this question has led
to more individuals reporting Medicaid coverage, it appears that the difference
between estimates of Medicaid coverage between the March CPS and CMS remains
sizable.
Figure 3. Ever Covered by Medicaid During 1998, U.S. Civilian
Noninstitutionalized Population
(in millions of individuals)
40
38.7
37.0
34.4
35
30
26.4
25
20
15
10
5
0
CMS
March CPS
SIPP
MEPS
Source: Congressional Research Service (CRS) using data from the Centers for Medicare and
Medicaid Services (CMS), the March CPS, SIPP, and MEPS. The CMS data cover the fiscal year
1998, while the other data are for the calendar year 1998. Efforts were made to increase the
comparability of these estimates by adjusting the CMS data to reflect only the noninstitutionalized
population. Three categories reflect services to the institutionalized Medicaid population: mental
health facility services (MHF), nursing facility services (NFS), and intermediate care facility-
mentally retarded services (ICF/MR). It is possible that individuals may receive more than one of
these services, and therefore simply subtracting all three categories of services may underestimate
the total noninstitutionalized Medicaid population. Based on conversations with CMS officials, the
CMS number was adjusted for Figure 3 by subtracting the number of recipients of NFSs and
ICFs/MR. However, as it is possible that residents of MHF may also receive one of the other two
previous services, these individuals remain in the estimate (accounting for approximately 125,000
of the 38.7 million reported by CMS).

CRS-12
Recall Period. Another issue that may explain some of the variation in the
uninsured estimates is the length of time respondents are asked to remember their
health insurance status (the recall period). Individuals who are asked to report their
status for a period of time might not accurately report changes in their health
insurance status over that period of time. The March CPS requires respondents to
recall their coverage over the longest time period (15 months), compared to the other
surveys. For example, respondents were asked in the March 2001 CPS about their
sources of health insurance coverage during the calendar year 2000. NHIS asks about
health insurance coverage during the previous 12 months, SIPP during the previous
4 months, and MEPS during the previous 3 to 5 months. The longer the amount of
time, the more likely that respondents may forget a source of coverage, particularly
if the duration of that coverage was short.
Additionally, some respondents, when asked to report their health insurance
status for the entire preceding year, may answer the health insurance questions based
on their current coverage. If true, this could explain why the March CPS’s full-year
uninsured estimates are higher than those from the other surveys but are similar to
their point-in-time estimates. However, aside from similar estimate values (as
illustrated in Figure 1) there is little evidence to support this argument. In fact,
according to the analysis by Census Bureau staff comparing the March CPS and SIPP,
the respondents in the March CPS “do not report annual health insurance coverage
information based on their current status rather than the previous year.”19 Instead, the
analysis found that the key cause for the differences between the surveys is that March
CPS respondents tended to underreport health insurance coverage from private
sources when compared to SIPP respondents.20
Longitudinal Design. Unlike the CPS and NHIS, the SIPP and MEPS re-
interview the same people over multiple years in order to track changes over time.
With respect to health insurance, therefore, the SIPP and MEPS have the advantage
of being able to analyze the duration of spells without insurance on a month-to-month
basis and over the entire duration of the panel. These longitudinal surveys do present
unique issues, however. With each wave of interviews, fewer people respond, so the
sample size and response rates shrink over the life of the panel. In the first wave of
the 1996 SIPP panel, the response rate was 92%, but by the final wave 4 years later,
the response rate was 65% of the original 1996 eligible sample. This may affect the
estimates of the uninsured if those who are uninsured are either more or less likely to
drop out of the survey than those who are insured. In other words, if over the life of
the panel we see the percentage of uninsured individuals dropping, this may be
because the number is truly dropping or because those who are uninsured are less
likely to continue responding and therefore be counted in the survey.
19 U.S. Bureau of Census. A Comparative Analysis of Health Insurance Coverage
Estimates: Data from CPS and SIPP, by Robert L. Bennefield.
20 The recent results from the Census Bureau’s paper on the verification question may lend
more credence to this statement. That paper stated that 89% of those who were initially
classified as uninsured but who ultimately reported coverage because of the verification
question revealed that they had private coverage.

CRS-13
The SIPP follows its respondents for 4 years, while MEPS follows its
respondents for 2 years. Because the MEPS study period is shorter than the SIPP,
MEPS does not experience a similar reduction in sample size. MEPS is also unique
because its panels (i.e., survey samples) overlap. Each year, MEPS is administered
to a new sample of individuals (who are drawn from the previous year’s NHIS
sample), and these individuals are followed for a period of 2 years. This allows a
number of estimates to be produced. For example, the 1996 MEPS panel provides
information on health care utilization and expenditures for calendar year 1996 and
1997, as the panels are followed for 2 years. In addition to the information provided
in 1997 by the 1996 panel, a new panel of individuals is introduced in 1997 for whom
information is also collected. Because information is available for both panels for
1997, they both contribute to a calendar year data file for 1997. By combining the
two samples, the total combined sample size increases. Therefore, attrition will be a
concern in analyzing data for the same panel of individuals (for example, looking at
2 years of information for the 1996 MEPS panel), as is the case with the 1996 SIPP.
However, the calendar year data, which combine the overlapping MEPS panels, are
valuable because the larger combined sample size enhances the reliability of estimates
produced on the uninsured.
One advantage of longitudinal analyses is following the changing composition
of households and families throughout the life of the panel. For example, suppose a
single mother and her two children were uninsured at the time of her first interview
for the 1996 SIPP. This mother would be interviewed 12 times the 4-year panel. In
that time, it is possible that she would marry, at which point her husband would
become part of her family. If her husband has health insurance, the mother and her
two children may then be covered under his health insurance. Therefore, as a result
of the change in family composition over the survey period (in this case, a marriage),
the health insurance status of the woman and her children has changed. However,
while panel surveys permit more types of analyses, they are also more complicated,
and care must be taken when using these surveys’ longitudinal capabilities. In order
to follow changes in health insurance status over time within families or households,
it is necessary to decide how the household and family changes will be handled.21
Full-year Analysis. The four surveys in this report can be used to count the
uninsured using three different time periods: the number of people uninsured for a full
year, at a point in time, and ever during the year. Each time period gives analysts a
slightly different perspective on the uninsured. The full-year estimate looks at those
who were chronically uninsured — those who were not enrolled in insurance at all
during the year. However, many may move in and out of the ranks of the uninsured.
Approximately twice as many people are ever uninsured during the year compared to
the number uninsured for the entire year, according to SIPP, NHIS and MEPS
estimates, as previously illustrated in Figure 1 and shown in Appendix B.22
Full-year analyses of the uninsured are also complicated by the presence of those
who were not eligible for the survey over the entire year, particularly infants (defined
21 This is not a major concern for analyses that are of individuals rather than families or
households.
22 The March CPS provides only full-year estimates.

CRS-14
here as those less than a year old). AHRQ economists exclude infants from their full-
year analyses using MEPS, as they are interested in a person’s unchanged uninsured
status over a 12-month period. However, the Census Bureau includes infants in its
estimate of the uninsured and in the formula to allocate funds to states for the SCHIP
program. The March CPS questionnaire flows in such a way that infants who were
uninsured for their entire lives, even if they were alive for only 1 month in the survey
year, would be counted as uninsured for a full year.23 Individual researchers need to
decide what is best for their purposes.
Discussion
The four surveys reviewed in this report have unique strengths and limitations,
resulting from differences in each survey’s purpose, sample, and questions. One
major concern about the March CPS has recently been addressed, with the decision
to include a question that requires respondents to verify that one or more household
members are uninsured. While the addition of this question has decreased the number
reported without health insurance, the March CPS estimates remain much higher than
those from the other surveys. Nevertheless, the March CPS continues to be the data
source of choice by most for estimates of the uninsured because it provides the most
recent full-year information and is representative of all 50 states and the District of
Columbia. However, its focus is not health insurance but work, income and poverty
statistics, and it does not delve deeply into issues of insurance coverage. In addition,
the March CPS can not provide estimates of those uninsured at a point in time during
the year or of those ever uninsured during the year; it permits estimates only of those
individuals uninsured for the entire year. As a result, the other surveys provide
additional useful detail related to people’s enrollment in health insurance and their
circumstances without it.
As policymakers develop proposals to reduce the number of uninsured, questions
continue to be raised that relate to characteristics of the uninsured. Given the
limitations of the March CPS, it becomes important to consider other sources for
health insurance information, such as those available in NHIS, MEPS and SIPP. For
analyses that relate health insurance status to health risk factors and general health
issues, NHIS supplies the most complete data. In addition, NHIS is able to link to
other surveys, such as MEPS and the National Survey of Family Growth (NSFG),
which allows researchers to examine a broader range of issues. While NHIS collects
more detailed information on health insurance coverage, the inability to produce
reliable estimates for all 50 states and the District of Columbia limits its usefulness in
evaluating proposals for the uninsured.
SIPP and MEPS both are useful in examining changes in health insurance and
other characteristics over time because of their longitudinal designs. SIPP is most
valuable in its capacity to relate people’s insurance status to their income, poverty
status, assets and liabilities, as well as their participation in government programs.
MEPS, on the other hand, supplies the most detailed information to relate insurance
23 As a result, one might expect the full-year estimate for infants to be closer to the point-in-
time estimate, compared to other age cohorts. Although our preliminary estimates bear this
out, an in-depth discussion of this point is beyond the scope of this report.

CRS-15
status to people’s use of and spending on health care. Of the surveys, MEPS also
asks the most comprehensive questions about the type and source of people’s health
insurance. However, these panel surveys that delve deeper into issues face higher
costs and longer interview periods with individuals, factors that often limit the size of
the survey sample. Both MEPS and SIPP have smaller sample sizes than NHIS and
CPS. Therefore, although these surveys may also produce representative estimates
of the U.S. population, their inability to produce reliable estimates of the 50 states and
the District of Columbia also hampers their usefulness in evaluating proposals for the
uninsured.
With data from these four surveys, it is possible to answer many questions about
the uninsured. As discussed in this report, the information collected among the four
surveys varies, but by exploring the various aspects covered in each survey
researchers are able to delve deeper into issues that affect the uninsured U.S.
population. Because the estimates among the surveys differ by so much, however,
the results should always be interpreted with some caution until the differences can
be reconciled. In addition, given the variation among states in their policies and
programs that affect the uninsured, the ability to calculate reliable estimates for each
of the 50 states and the District of Columbia remains important. To this end, the
March CPS remains the most widely cited estimate of the uninsured because of its
ability to calculate reliable state-level estimates.

CRS-16
Appendix A. Comparison of Characteristics of Federally Administered Surveys
March supplement
SIPP
NHIS
MEPS
to the CPS
Survey overview
Primary Focus of
To provide information on
To provide information on
To provide information on
To provide information
Survey
labor force characteristics
income, program
the health of the U.S.
about the use of and
participation
population
expenditures for health
care
Survey Design
Cross-sectional (i.e., not
Longitudinal — For 1996
Cross-sectional
Longitudinal — follows
designed for analyses of the
panel, followed each
each household for 2 years,
same person/household from
household in survey for 4
with new households
year to year)
years. Subsequent panels to
chosen annually
be 3 years in length
(overlapping panels).
Sample drawn from
previous year’s NHIS
Universe
U.S. civilian
U.S. civilian
U.S. civilian
Same as NHIS
noninstitutionalized
noninstitutionalized
noninstitutionalized
population and those
population and those
household population
members of the Armed
members of the Armed
Forces with a civilian adult
Forces stationed locally and
in the household
living in sample unit
Geographic Area
50 states and the District of
Same
Same
Same
Columbia.
Most Recent Data
2000 data from March 2001
November 1999 to February
2000 (provisional release)
2000 point-in-time data;
supplement
2000 (Wave 12 of 1996
1998 full-year, month-to-
Panel)
month data
How often is data
Annually, typically in
Periodically, with first wave
Annually, though at no
Periodically, with various
released?
September
of 2001 panel data to be
particular time of year
data files released
released early 2002
throughout the year
History
Health insurance questions
Annual overlapping panel
Annually, beginning in
Annually, beginning in
added in March 1980
surveys ran from 1984 to
1957
1996. Historically, two
survey, though consistent
1993; in 1996, 4-year panel;
surveys (1977 NMCES and
data is only available
new panels to be 3 years
1987 NMES) are
beginning with 1988
considered predecessors
Administering Agency
Census Bureau and Bureau
Census Bureau
National Center for Health
Agency for Healthcare
of Labor Statistics (BLS)
Statistics (NCHS)
Research and Quality
(AHRQ), partnering with
NCHS

CRS-17
March supplement
SIPP
NHIS
MEPS
to the CPS
Survey details (Analytic Issues)
Nationally
Yes
Yes
Yes
Yes
representative
estimates?
State-level estimates?
Yes
No
Largest states only
No
Respondent(s)
One person, usually the
All household members 15
All adult members of the
The one person in the
individual who owns or rents
years old and over, if
household 17 years of age
h o u s e h o l d m o s t
the housing unit, for
possible; otherwise proxy
and over who are at home
knowledgeable about the
everyone in the household. If
response is permitted.
at the time of the interview
h o u s e h o l d ’ s m e d i c a l
t h a t p e r s o n i s n o t
A d u l t ( s ) p r o v i d e s
are invited to participate
s i t u a t i o n s p r o v i d e s
knowledgeable regarding the
information on any children
and to respond for
information for everyone in
other adults in the household,
in the household.
themselves. For children as
t h e h o u s e h o l d . F o r
attempts are made to contact
well as adults not at home
information difficult for
them.
during the interview,
respondent to know, MEPS
information is provided by a
obtains permission to
responsible adult family
collect information from
member (18 years of age
their employers, providers,
and over) residing in the
and plans.
household.
Information collected
Yes
Yes
Yes
Yes
for everyone in
household?
Definition of
All the persons who occupy a
All the persons for whom the
An occupied dwelling unit
Same as NHIS, except that
Household
house, an apartment, or other
sampled address is their
with one or more civilian
college-aged students
group of rooms or a room,
usual place of residence (i.e.,
members.
living away from home
when occupied as separate
where a person normally
during the school year were
living quarters.
lives and sleeps). Newcomers
interviewed at their place of
to the household during the
residence for the NHIS but
panel are included. Those 15
were identified by and
and older who move from the
linked to their parents’
household are contacted and
household for MEPS.
that household is considered
another household, with all
residents interviewed.
Sample Size
Approximately 50,000
Approximately 37,000
Approximately 42,000
Approximately 12,000
eligible households (2001).
eligible households (1996
eligible households (1998).
eligible households (2000
panel).
point-in-time file).

CRS-18
March supplement
SIPP
NHIS
MEPS
to the CPS
Units of Analysis
! Person
! Person
! Person
! Person
! Family
! Family
! Household
! Family
! Household
! Household
! Condition
! Medical event
! P r o g r a m
! Doctor visit
! Condition
participants
! Hospital stay
! Job
Time Period
15 months
4 months
Up to 12 months
3 to 5 months
Respondent Asked to
Recall
Populations
Hispanics
Low-income individuals
Blacks and Hispanics
Blacks and Hispanics. Panel
Oversampled
2 (1997) also oversampled
the functionally impaired,
children with limited
a c t i v i t y , i n d i v i d u a l s
predicted to have high
medical expenditures, and
those predicted to have
family income less than
200% of the poverty level.
Health Insurance Information
Direct question
Yes
No
Yes
No
to confirm
(beginning in March 2000)
(however, questions that
(beginning in 1997)
(however, questions that
restate respondents’
restate respondents’
uninsured status?
insurance status may elicit a
insurance status may elicit
correction)
a correction)
Measurable Periods
! An entire year
! Any month
! At time of survey
! Any month
Without Insurance
! At time of survey
! Any part of year
! At time of survey
(point-in-time)
! An entire year
(point-in-time)
! Ever during year
! Ever during year
! An entire year
! An entire year
! For entire panel
! For entire panel
(e.g., 1996-1999)
(e.g., 1998-1999)
Able to follow an
No
Yes
No
Yes
individual’s changes in
(by month)
(by month)
insurance status?
Source: Congressional Research Service (CRS).

CRS-19
Appendix B. Uninsured U.S. Civilian Noninstitutionalized
Population, from Federally Administered Surveys, 1998
March
Supplement
to CPS
SIPP
NHIS
MEPS
Uninsured for
43.6 million
21.2 million
25.8 million
31.5 million
full yeara
(16.3%)
(7.9%)
(9.7%)
(11.8%)
Uninsured at time of
N/A
39.2 million
39.5 million
42.3 million
survey (point in time)
(14.6%)
(14.7%)
(15.8%)
Ever uninsured during
N/A
55.2 million
50.0 million
60.3 million
the yeara
(20.7%)
(18.9%)
(22.7%)
Source: Congressional Research Service (CRS) analysis of March CPS, SIPP, NHIS, and MEPS.
Notes: The percentages represent the share of the U.S. civilian noninstitutionalized population
without health insurance. As discussed in the memorandum, the March CPS does not support
estimates of the number uninsured at the time of the survey or the number ever uninsured during the
year. For the point-in-time estimate, SIPP data are from March 1998, NHIS data are from interviews
throughout the year, and MEPS data are from the first half of 1998. For SIPP and MEPS, the
percentage of uninsured for the full year and at any time excludes those who did not provide data for
all 12 months.
a Excludes infants (i.e., those less than a year old). Although the commonly cited March CPS
estimates of the full-year uninsured and CRS’s own estimates include infants, infants are
excluded from our full-year analysis here for purposes of comparability. Given differences
among the surveys, it was not possible to calculate comparable estimates that include infants
among the surveys. As a result, infants were dropped in all of the surveys’ full-year analyses.
For similar reasons, infants were also excluded from the any-time-in-year estimates. Similar
concerns and complications do not exist for point-in-time estimates, so infants are included
in those estimates.