Department of Health and Human Services 

Office of the Assistant Secretary for Planning and Evaluation 

http://aspe.hhs.gov 

ASPE

ISSUE BRIEF

UNDERSTANDING ESTIMATES OF THE UNINSURED:

PUTTING THE DIFFERENCES IN CONTEXT

ACKNOWLEDGMENTS: ASPE would like to thank those reviewers at the U.S. Census Bureau, Agency for 

Healthcare Research and Quality (AHRQ), and the National Center for Health Statistics (NCHS), whose helpful 

comments and expertise contributed greatly to this document. 

INTRODUCTION

The number of uninsured individuals in the United States has been an important policy concern 

for many years. The four federal surveys that are major sources of data on the uninsured have 

played an important role in informing policy on this issue. These surveys: the Current 

Population Survey (CPS), the Medical Expenditure Panel Survey (MEPS), the National Health 

Interview Survey (NHIS), and the Survey of Income and Program Participation (SIPP) can each 

provide useful estimates of the number of uninsured individuals during a particular period of 

time, and in some cases, at a point in time. Both MEPS and SIPP also provide information on 

changes in the insurance status of individuals over time. 

Why are there so many surveys which measure health insurance? Actually, each of the surveys 

measures far more than insurance status. Surveys which measure insurance status actually 

address multiple topics and each has different strengths. The CPS, primarily a survey related to 

employment, has the best source of income data and is large enough to yield state data. The 

NHIS, the broadest survey of health, is useful in analysis of disease prevalence and health risks 

in the population. The MEPS provides the greatest depth regarding expenditures and utilization. 

The SIPP can measure monthly changes in income and employment status. Because health 

insurance is so important, it is collected on each of these surveys so that it can be related to other 

content that is unique to those surveys. What follows is a more complete description of each 

survey. 

GENERAL SURVEY DESCRIPTIONS

CPS: Conducted by the U.S. Census Bureau, the CPS is a monthly survey designed to collect 

data on the population’s employment status. The Annual Social and Economic Supplement to 

the CPS asks additional questions about health insurance coverage. The CPS data are widely 

used because the CPS has the largest household survey sample size, is designed to produce 
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ASPE Office of Health Policy September 2005 

credible state-level estimates (less populous states may require 3 years of CPS data to produce 

such estimates), is available on a timely basis, and provides information on coverage rates for 

socio-demographic subgroups of the population. However, as with most surveys, the CPS has 

been subject to questions regarding over- and under-reporting. Specifically, it is thought that the 

CPS over-counts the number of individuals who have been uninsured for an entire year, possibly 

because respondents answer based on current, rather than previous, coverage status. In addition, 

Medicaid coverage status is likely under-reported.1

MEPS: Administered by the Agency for Healthcare Research and Quality (AHRQ), MEPS is a 

two-year panel survey that collects information about health care use and expenditures, and the 

dynamics of health insurance and delivery systems. MEPS data on an individual’s health 

insurance status are collected several times a year and have the capacity to produce a variety of 

estimates of health insurance coverage, including point-in-time, monthly, and annual estimates. 

As a longitudinal survey, MEPS is able to examine health insurance dynamics, including 

changes in coverage and spells without coverage. MEPS data can be used to analyze the 

relationship between insurance status and a variety of individual and household characteristics, 

including use of and expenditures for health care services. Data on sources of payment for 

health care are collected and can be used to add greater accuracy and precision to the insurance 

status estimates. The MEPS sample is smaller than the CPS and NHIS samples and does not 

contain a representative sample from each state, and thus cannot be used to make state-level 

estimates. 

NHIS: Administered by the National Center for Health Statistics (NCHS), the NHIS is a 

continuous household interview survey of the civilian noninstitutionalized population. The 

NHIS collects detailed information on insurance status, type of coverage, measures of health 

status, health care utilization, and socio-demographic characteristics of members of sampled 

households. It provides estimates of the number of persons uninsured at a point-in-time, 

uninsured for a full-year, and ever uninsured during the year. The NHIS also collects data on 

illnesses, injuries, activity limitations, chronic conditions, health behaviors, and other health 

topics, which can be linked to characteristics such as insurance status. Starting in 2001, NCHS 

began publishing quarterly early reports that provide insurance data only six months after a given 

quarter, which is the most up-to-date data of any survey. Like MEPS and SIPP, the NHIS cannot 

provide reliable state estimates, except in the largest states. 

SIPP: Also administered by the Census Bureau, SIPP is a 36- to 48-month longitudinal panel 

survey that collects information on health insurance status, income, labor force activity and 

federal program participation. As a longitudinal survey, SIPP data provide the capacity to 

examine the dynamics of health insurance. It measures the duration of spells without health 

insurance and provides health insurance estimates for various time periods, such as point-in-time, 

monthly, annual, or over the full panel. As part of the core data collected in the SIPP, health 

insurance data can also be linked to other sections of the survey, such as utilization of health care 

services, child well-being, and disability. The SIPP sample is smaller than the CPS and NHIS 

samples and does not contain a representative sample from each state, and thus cannot be used to 

make state-level estimates.2
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ASPE Office of Health Policy September 2005 

THE ESTIMATES

The following table presents the uninsured estimates from the major surveys discussed above. 

As the table shows, there is significant variability in the number of uninsured from survey to 

survey. 

Table 1. Uninsured Estimates from Various National Surveys 

Method of Estimate 

Survey 

Most 

Recent 

Year 

Uninsured For 

Full Year Point in Time 

Uninsured Ever 

During the 

Year 

Current Population Survey 

(CPS) 2004 

45.8 million 

15.7% N/A N/A 

Medical Expenditure Panel 

Survey (MEPS)* 2003 

33.7 million 

13.6% 

48.1 million 

19.0% 

(2004) 

62.9 million 

25.4% 

Survey of Income and 

Program Participation (SIPP) 2001 

18.9 million 

 6.8% 

38.7 million** 

14.0% 

66.5 million 

24.0% 

National Health Interview 

Survey (NHIS) 2004 

29.2 million 

10.1% 

42.1 million 

14.6% 

51.6 million 

17.9% 

 Note: 

 N/A = Survey does not capture this dimension. 

MEPS point in time refers to the period from Jan. 1 through the interview date, which is 5 

months on average. 

 

 *non-elderly 

 **Estimate for March 2001. 

SOME REASONS FOR DIFFERENCES AMONG ESTIMATES

As is clear from the above table, the number of uninsured can vary widely from survey to survey. 

There are many variables that contribute to variance in survey estimates, such as general survey 

design and data collection methods. Below is a discussion of the major reasons that most likely 

explain the variance in uninsured numbers.3

 (See Table 2 at the end of this paper for a summary 

of key survey differences) 
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1. Survey Design Differences 

• The Length of Time Uninsurance is Measured. The most obvious variable in the 

above table is the length of time used to measure an insured spell. This time dimension is 

particularly important because health insurance status is relatively volatile and estimates 

will vary with the length of time coverage is examined. As more time passes, more 

people will experience a lapse in health coverage. Thus, the total number of people who 

experience a period of uninsurance over the course of an entire year (referred to as everuninsured) will be greater than the number of uninsured at a given point in time (such as 

the time of the survey interview). Furthermore, of the people who experience a lapse in 

coverage, most will have short spells without coverage while fewer will have spells that 

last a year or longer. Therefore, the full-year uninsured estimate (the number of people 

who were uninsured for a full twelve months) will generally be smaller than the point-intime and ever-uninsured-during-the-year estimates. Therefore, when analyzing survey 

results, it is important to understand what period of time the data are measuring. 

• The CPS Controversy. Because the CPS is the most widely cited source of insurance 

coverage estimates, it is important to note a fact that can be readily seen in Table 1: the 

CPS estimate of the full-year uninsured in 2004, 45.8 million, is far higher than the fullyear estimates provided by other surveys. In fact, the CPS full-year estimate looks more 

in-line with other surveys’ point-in-time estimates. Given this higher estimate, many 

analysts hypothesize that a number of respondents to the CPS actually provide point-intime information (i.e., information about their status at the time they are participating in 

the survey (March) or about their status at the end of the previous calendar year) despite 

the fact that the survey questions ask about the entire previous calendar year. How many 

respondents misinterpret the relevant questions in this manner, if any, is unknown, but 

this uncertainty may make it more difficult to interpret and compare data on the 

uninsured. 

• Recall Periods. A second important difference between surveys is the length of the 

recall period. Most survey methodologists believe that the longer the recall period, the 

less accurate the answers will be to questions about status in the past. For the CPS, the 

respondent is required to remember the previous 13 to 15 months since the health 

supplement is conducted in March following the calendar year to which the questions 

pertain. All other surveys except the CPS ask about coverage at the time of the interview. 

When determining health insurance status over the year, the recall period is up to 3-5 

months for MEPS, up to four months for SIPP, and up to 12 months for NHIS. 

• How Insurance Is Defined. The way insurance is defined in each survey determines 

who is categorized as insured. MEPS defines private insurance as coverage for hospital 

and physician services, thereby eliminating serious and dread disease, workers 

compensation, accident, and disability policies from counting as coverage. As well, 

MEPS does not include single service plans (such as dental plans) as private insurance 

except for single service hospital coverage. CPS and SIPP instruct interviewers to not 

count single service plans as private insurance, but some single service coverage may get 

misreported as comprehensive coverage. 
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• How Respondents Are Asked about Insurance. To determine if a person is uninsured, 

surveys use one of two methods: they ask respondents directly if they lack insurance 

coverage or they classify individuals as uninsured if they do not affirmatively indicate 

that they have coverage. CPS and NHIS directly ask respondents whether they lack 

insurance coverage, while MEPS and SIPP do not. While the difference in the two 

approaches may seem subtle, some researchers believe that surveys which fail to verify a 

lack of coverage will over count the uninsured. The theory is that this verification 

question prompts some people who did not indicate any insurance coverage to rethink 

their status and indicate coverage that they previously forgot to mention. Because of this 

issue, NHIS added a verification question in 1997 and CPS added this component in 

2000. It should be noted that the effect of this question has not been uniform in all the 

surveys which use it: in several privately funded surveys that are also sources of data on 

the uninsured, the National Survey of America’s Families (NSAF) and the Community 

Tracking Study (CTS), asking the verification question has resulted in lower uninsured 

estimate, while the effect on the 1997 NHIS estimates appeared to be negligible. 

• Focus of Survey. Each survey has a different focus and asks questions about health and 

health insurance with differing degrees of detail. The CPS is primarily a labor force 

survey, with some health insurance questions added onto the end. In contrast, the other 

surveys have a clearer focus on health and health insurance. While difficult to verify, 

some believe that a focused health survey is more likely to elicit accurate responses 

regarding health insurance because survey questions on health status and health care 

utilization will lead to better recall regarding the insurance coverage that was needed to 

pay for treatment. For example, when comparing CPS and SIPP, Census Bureau officials 

have hypothesized that one of the reasons SIPP may be better suited to measure health 

insurance status is that the health questions are more detailed and that they actually ask to 

see insurance cards.4

 Similarly, the MEPS survey asks a number of questions about 

health status and medical care utilization before asking about insurance status. 

2. Differences in Timeliness of Data 

Because of different lag times between data gathering and data availability, it is often the case 

that results cited at the same time actually reflect data gathered in different years. For example, 

the most recent data available from the CPS were collected in March of 2005 and reflect 2004 

coverage rates, the most recent data available from the NHIS were collected in 2004 and reflect 

2004 coverage rates, and the most recent MEPS data on full year insurance coverage were 

collected throughout 2003 and into 2004, reflecting 2003 coverage rates. MEPS data for point in 

time insurance coverage is collected in early 2004 and released in June of 2005. Therefore, 

when comparing estimates of the uninsured, it is important to note which year the survey covers. 

3. Data Reporting 

Surveys often make available public data files which contain the raw survey data, allowing 

organizations to publish their own uninsured estimates. Various researchers and organizations 

have adopted different methods to adjust the raw survey data to meet their specific needs. For 
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ASPE Office of Health Policy September 2005 

example, some will adjust the CPS to account for Medicaid under-reporting while others will 

adjust income measures to better simulate eligibility for government programs. These researcherspecific adjustments can result in different estimates derived from the same survey, particularly 

for population subgroups, such as the number of uninsured below poverty or the number of 

uninsured children eligible for Medicaid. Therefore, the uninsured estimates from CPS 

published by the U.S. Census Bureau have at times differed from those published by other 

organizations because of differences in the way the data are compiled.5

 

Table 2. Selected Differences Between Surveys: Uninsured Estimates 

Survey Type of 

Estimate Produced 

Respondent 

Recall Period 

Sample Size Most 

Recent 

Data 

From 

Source of 

Data on 

Health 

Insurance 

Dynamics? 

Source of 

State 

Estimates? 

CPS Full Year Uninsured Prior 13 to 

15 months 

210,648 

(98,664 

households) 

2004 no yes 

SIPP Full Year Uninsured 

Point in Time 

Ever Uninsured 

 During the Year 

Prior 4 months 60,000 

(26,000 

households) 

2001 yes no 

NHIS Full Year Uninsured 

Point in Time 

Ever Uninsured 

During the Year 

Time of interview 

and prior 

12 months 

94,460 

 (36,579 

households) 

2004 no only for 

large 

 states 

MEPS Full Year Uninsured 

Point in Time 

Ever Uninsured 

During the Year 

Time of interview 

and prior 

3-5 months 

34,215 

(12,860 

households) 

2003 yes no 

CONCLUSION

It is clear that the estimates of the uninsured may vary depending upon the data source and data 

adjustments. The decision of which survey to use may depend on the purpose of the analysis. 

For credible state-level estimates, the CPS is the only source for all 50 states. Larger sample 

sizes enable CPS and NHIS to produce more reliable estimates for subgroups of the population 

(i.e. children, low-income workers, etc.). MEPS and SIPP are the best sources for examining 

changes in individuals’ insurance status over time and NHIS, MEPS, and SIPP can provide 

point-in-time estimates of the uninsured. 

 

Despite the differences that distinguish these surveys, the estimates derived from the surveys in 

total paint a relatively consistent picture of health coverage rates in the United States. While the 

overall number of uninsured varies from survey to survey, rising and falling trends in the 

estimates are consistent between the surveys. In addition, uninsured estimates among population 

subgroups also tend to be consistent. For example, all surveys show that the uninsured are 

concentrated among low-income individuals, and all surveys show that Hispanics have the 
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ASPE Office of Health Policy September 2005 

highest uninsured rate among the racial and ethnic groups. Therefore, the surveys can still help 

policy makers tackle critical policy concerns related to health insurance coverage. 

NOTES

 1. Previous version of this memo contained a section called Medicaid Adjustments, which 

explained how some surveys, most notably the CPS, assign Medicaid coverage to individuals 

who also receive welfare assistance even if the survey respondents do not report Medicaid 

coverage. Historically, this assignment of Medicaid, called an imputation, was done because a 

significant percentage of respondents who indicated they had AFDC (the pre-welfare reform 

cash assistance program) failed to report Medicaid, even though people who received AFDC 

automatically received Medicaid by law. The Census Bureau continues to assign Medicaid to 

those who indicate they receive cash assistance through the new TANF program, though 

Medicaid and welfare eligibility have been delinked. Census continues to make this imputation 

because it appears States still provide Medicaid to the vast majority of TANF cash recipients. It 

was decided to drop this section from the memo because some surveys which do not assign 

Medicaid to welfare recipients find a higher number of Medicaid enrollees than the CPS. 

Therefore, it is no longer clear that the Medicaid imputation is a major factor in explaining why 

survey estimates vary. Nonetheless, Medicaid imputation is an important survey design 

difference that analysts should be aware of. 

2. In 1996, SIPP started asking about coverage during the month of the interview, in addition to 

asking about coverage during the months prior to the interview. In 1994, it only asked about 

coverage during the months prior to the interview. 

3. Also affecting estimates are factors such as the target population (e.g. U.S. civilian 

noninstitutionalized), the survey mode (face-to-face interview vs. telephone), survey response 

rates, and nonresponse rates to health insurance coverage questions, which are not discussed in 

this paper but should be noted. 

4. Lewis, Kimball, Marilyn Ellwood and John Czajka. Counting the Uninsured: A Review of the 

Literature. Assessing the New Federalism: Occasional Paper Number 8. July 1998. Pg 15. 

 http://newfederalism.urban.org/html/occ8.htm 

5. CPS switched to a computer-assisted personal interview (CAPI) system in 1994. Also, in 

1995, changes were made to the health section of the CPS March supplement such as reordering 

the questions and changing some questions about private insurance. In 1997, major changes 

were made to the NHIS, such as no longer imputing Medicaid to AFDC recipients, switching to a 

CAPI system and redesigning the survey to include more specific health insurance questions. 

Survey instrument adjustments need to be taken into account when looking at a historical time 

series because new survey designs can produce changes in estimates that may not reflect actual 

changes in the insurance coverage of the population. 
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ASPE Office of Health Policy September 2005 

ADDITIONAL INFORMATION

Websites 

Agency for Healthcare Quality and Research, Medical Expenditure Panel Survey: 

 http://www.meps.ahrq.gov 

Center for Studying Health System Change, Community Tracking Study: 

 http://www.hschange.com/tracking.html 

 

Economic Research Initiative on the Uninsured, University of Michigan: 

 http://www.umich.edu/eriu 

National Center for Health Statistics, National Health Interview Survey: 

 http://www.cdc.gov/nchs/nhis.htm 

Urban Institute, National Survey of American Families: 

 http://newfederalism.urban.org/nsaf/index.htm 

U.S. Census Bureau. Health Insurance Data, Current Population Survey (CPS) and Survey of 

Income and Program Participation (SIPP): 

 http://www.census.gov/hhes/www/hlthins.html 

U.S. Census Bureau, Survey of Income and Program Participation Main Page: 

 http://www.sipp.census.gov/sipp 

 

U.S. Census Bureau, Current Population Survey Main Page: 

 http://www.bls.census.gov/cps/cpsmain.htm 

Articles 

Bennefield, Robert. A Comparative Analysis of Health Insurance Coverage Estimates: Data 

From CPS and SIPP. Presentation at the 1996 Joint Statistical Meetings of the American 

Statistical Association, 1996c. 

Bennefield, Robert. Who Loses Coverage and for How Long? Current Population Reports, P70-

54. Washington, DC: Census Bureau, May 1996b. 

Bhandari, Shailesh, 2004, "People with Health Insurance: A Comparison of Estimates from two 

Surveys", Survey of Income and Program Participation Working Paper 243, U.S. Census Bureau. 

 http://www.sipp.census.gov/sipp/workpapr/wp243.pdf 

Center for Studying Health System Change and Mathematica Policy Research, Inc. Estimates of 

Health Insurance Coverage in the Community Tracking Study and the Current Population 

Survey. Technical Publication 16, November 1998. 

 http://www.hschange.org/CONTENT/172/ 
ASPE Issue Brief Page 9

ASPE Office of Health Policy September 2005 

Kenney, Genevieve et al. The National Survey of America’s Families: An Overview of the 

Health Policy Component. Inquiry. Fall 1999. Vol. 36, pg. 353-362. 

Lewis, Kimball, Marilyn Ellwood and John Czajka. Counting the Uninsured: A Review of the 

Literature. Assessing the New Federalism: Occasional Paper Number 8. July 1998. Pg 15. 

 http://newfederalism.urban.org/html/occ8.htm 

Monheit, Alan. Underinsured Americans: A Review. Ann. Rev. Public Health. 1994. Vol. 15, 

pg. 461-85. 

Rajan, Shruti, Stephen Zuckerman, and Niall Brennan. Verifying Insurance Coverage: Impact 

on Measuring the Uninsured with NSAF. Assessing the New Federalism, The Urban Institute. 

August 1999. 

Short, Pamela. Counting and Characterizing the Uninsured, Economic Research Initiative on the 

Uninsured, ERIU Working Paper 2, Dec. 4, 2001. 

 www.umich.edu/~eriu/pdf/wp2.pdf