Linking children from the National Health and Nutrition 

Examination Survey to Medicaid enrollment and claims data 

Lisa B. Mirel1, Cordell Golden2, Gloria Wheatcroft2, Alan E. Simon2, and Kenneth C. 

Schoendorf2 

1National Center for Health Statistics, Division of Health and Nutrition Examination Surveys, LMirel@cdc.gov 

2National Center for Health Statistics, Office of Analysis and Epidemiology 

Introduction 

The National Health and Nutrition Examination Survey (NHANES) is a nationally representative survey that 

collects information on the health and nutritional status of adults and children in the United States, including 

information on health insurance coverage. Recently the 1999-2007 Centers for Medicare and Medicaid Services 

(CMS) Medicaid Analytic eXtract (MAX) files were linked to the 1999-2004 NHANES. This linkage was part of 

an interagency agreement between the National Center for Health Statistics (NCHS), CMS, and the U.S. Department 

of Health and Human Services (HHS) Assistant Secretary for Planning and Evaluation (ASPE), and the Social 

Security Administration (SSA). The linked data are available in the NCHS Research Data Center. The results of 

this study offer preliminary insights into opportunities to improve the collection of program participation 

information in a self-reported survey. 

For this record linkage, NCHS obtained records for its survey respondents who had Medicaid records in the CMS 

data files. NHANES participants were defined as linkage eligible if sufficient identifiers were obtained at the 

interview and records from CMS could be sought. This paper will assess concordance between a participant’s report 

of Medicaid enrollment at the time of their household interview in NHANES compared with the presence of a 

Medicaid enrollment record in the MAX files for children less than 18 years. We will compare the prevalence and 

characteristics of the concordant and discordant pairs. These results will be discussed in the context of challenges 

encountered in linkage and accurately collecting self-report of Medicaid enrollment in the survey. 

Background on NHANES 

NHANES is a continuous survey (1999-present) that includes an interview in the household followed by an 

examination in a Mobile Examination Center (MEC). NHANES is a representative cross-sectional sample of the 

U.S. civilian, non-institutionalized population that is selected using a complex, multistage probability design. 

Detailed descriptions of the NHANES sample design have been described elsewhere and only will be summarized 

here (Curtin et.al, 2012). Participants can be any age. An adult proxy responds to the majority of the questionnaire 

for children who are 16 years or younger. 

The NHANES household interview contains a section which includes sample participant level interview data on 

health insurance coverage. One reference person in each family responds for each individual sample participant in 

the family. The reference person or responsible adult may or may not be a sample participant. Multiple families in 

the same household may be selected as participants in the study. However, this section of the family interview is 

asked for each individual sample participant and not for the family as a whole. 

The initial set of questions in the health insurance section determined which sample participants had health 

insurance coverage. For those who had health insurance coverage, respondents were then asked “What kind of 

health insurance do you have?” The respondent was shown a hand card and asked to choose one from the list which 

included both Medicaid and Children’s Health Insurance Program (CHIP). For 1999-2004 NHANES, the public-use 

available data reports Medicaid and CHIP together for the response. This was changed after 2004. 

Background Medicaid/CHIP 

Detailed information on Medicaid and the CHIP can be found on the CMS website (http://www.medicaid.gov/). 

Briefly, Medicaid is a needs based entitlement program administered by CMS in collaboration with each state to 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

provide health care coverage to some vulnerable populations in the United States, including low-income children, 

and the aged or disabled poor. The MAX files are research oriented Medicaid claims files. The eligibility for 

Medicaid varies by state. There is mandatory eligibility for pregnant women with a family income below 133% of 

the HHS poverty guidelines (this is sometimes referred to as the federal poverty level), children under 6 years of age 

with family income below 133% of the HHS poverty guidelines, and children aged 6-18 years of age with family 

income below 100% of the HHS poverty guidelines. 

CHIP is a state based program offered to children and pregnant women whose incomes are too high to qualify for 

Medicaid eligibility. CHIP funds are capped with each state receiving a set allotment. The federal government 

matches state funding for CHIP. States receive higher federal funding matching rates for CHIP than Medicaid. Of 

particular note for this project, the MAX files are incomplete on CHIP because states have the option of not 

reporting stand-alone CHIP program data to the Medicaid Statistical Information System (MSIS) from which the 

MAX files are created. 

Methods 

For this analysis, we used the 1999-2004 NHANES data. NHANES participants were eligible for linkage if they did 

not refuse to provide a social security number (SSN) at the NHANES interview and provided sufficient personally 

identifiable information (PII) for linkage. SSNs for these participants were verified at SSA and then sent to CMS 

where data were extracted based on exact matches for SSN, gender and month and year of birth. Eligibility for 

linkage for a NHANES sample participant to obtain a CMS administrative record was independent of the 

questionnaire responses to the health insurance question and independent of a participant’s Medicaid eligibility. 

Our main research goal was to assess the reliability and accuracy of the NHANES Medicaid response as compared 

to the matched CMS records. Concordance was defined as having Medicaid/ CHIP reported as a health insurance 

source in NHANES and having a CMS administrative record within a month of the NHANES interview. The 

presence of a CMS administrative record within a month of the NHANES interview indicated active Medicaid 

enrollment status. The one month time period was chosen based on how the question was asked in NHANES which 

implied current enrollment. Records were also considered concordant if there was no report of Medicaid/ CHIP 

coverage in NHANES and no match to the CMS administrative records for Medicaid enrollment. Discordance was 

defined as a report of Medicaid/ CHIP in NHANES without CMS evidence of enrollment, or, conversely, no report 

of Medicaid/ CHIP in NHANES while actually being enrolled according to the MAX files. 

The first step of the analysis was to examine the overall analytic sample breakdown (Figure 1) and assess the overall 

matched results using weighted percentages, using the NHANES interview weights. We then looked at the 

percentage of concordance and discordance given the response to the Medicaid/CHIP question in NHANES. The 

denominator for these analyses included all linkage eligible sample participants. Socio-demographic characteristics 

were also examined, including the ratio of family income to the poverty index, household size, gender, age and race 

and Hispanic origin, for the concordant and discordant pairs. 

A t-test was used to compare means and the Chi-square test of independence was used to assess statistically 

significant differences between demographic characteristics and concordance or discordance of Medicaid enrollment 

according to the MAX files and the interview reporting of type of health insurance coverage, using a two-sided pvalue <0.05. Variances were estimated using Taylor Series Linearization, accounting for the complex design. All 

statistical analyses were performed using SAS (version 9.2, SAS Institute Inc., Cary, NC) and SAS-callable 

SUDAAN (version 10.0, Research Triangle Institute, Research Triangle Park, NC) software. 

Results 

From the 1999-2004 NHANES there were 11,586 participants aged less than 18 years who had a response to the 

type of health insurance question in NHANES (less than 1% were missing a response, n=65). About 84% of these 

participants were eligible for linkage (i.e., they did not refuse to provide a SSN at the NHANES interview, they 

provided sufficient PII for linkage and their SSN was verified at SSA). Of the 9,766 eligible for linkage, 4,242 

participants matched to the Medicaid file within 1 month of the NHANES interview (Figure 1). 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1. Analytic sample for 1999-2004 NHANES participants aged less than 18 years linked to CMS MAX file 

(note: all percentages in figure are unweighted) 

NHANES sample participants 

aged <18 years with a response to 

the type of health insurance 

question in NHANES: n=11,586 

Ineligible for linkage: 

n=1,820 (15.7%) 

Linkage eligible1 : 

n=9,766 (84.3%) 

Linked within 1 month of NHANES 

interview: 

n=4,224 (43% of linkage eligible) 

Did not link to a CMS MAX file at 

any time: 

n=3,888 

Linked to a CMS Max file but 

outside of the 1 month window: 

n=1,654 

1 Linkage eligible means they did not refuse to provide a SSN at the NHANES interview, they provided sufficient 

personally identifiable information for linkage and their SSN was verified at SSA. 

There are several different ways to assess the matched results. First we assessed the overall concordant and 

discordant weighted percentages for the linkage for all linkage eligible participants, n=9,766. The overall 

concordance, using weighted percentages was 89% (24%, n=3,101, said “Yes” in NHANES and linked within one 

month of the NHANES interview to the CMS MAX file and 65%, n=5,057, said “No” in NHANES and did not link 

to the CMS MAX file within 1 month of the NHANES interview or at any time). 

Table 1. Unweighted sample sizes for linkage eligible NHANES sampled participants, comparing response to 

NHANES question about Medicaid/CHIP to linkage status to the CMS MAX file. 

Linked to the CMS 

MAX within 1 month of 

NHANES interview 

Did not link to the CMS 

MAX file within 1 

month or at any time 

Total 

“Yes” to question in 

NHANES about 

Medicaid/ CHIP health 

insurance coverage 

3,101 485 3,586 

“No” to question in 

NHANES about 

Medicaid/ CHIP health 

insurance coverage 

1,123 5,057 6,180 

Total 4,224 5,542 9,766 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Of the respondents who said “No” to the question in NHANES, using weighted percentages,11% (1,123/6,180) 

linked within 1 month of the NHANES interview (making the agreement level of those who said “No” and were not 

linked 89%). Of the respondents who said “Yes” to the question in NHANES about Medicaid/CHIP health 

insurance coverage, using weighted percentages,12% (485/3,586) were not found in CMS MAX file (88% 

agreement among those who said “Yes” and linked within 1 month of the NHANES interview). 

For those who reported Medicaid/CHIP coverage in NHANES but were not linked to the CMS MAX database they 

had a higher (p-value=0.002) mean income to poverty threshold ratio (mean=1.2, standard error=0.07), compared to 

those who reported Medicaid/CHIP and were linked (mean=1.0, standard error=0.03). The discordance group who 

responded “Yes” in NHANES but did not link to the CMS MAX file may be because these participants were 

receiving CHIP health insurance coverage and the MAX files are incomplete on CHIP data. The ratio of family 

income to poverty threshold of 1.2 for the discordant pair actually suggests they would be eligible in most states for 

CHIP, as income eligibility is slightly higher for CHIP than Medicaid. Table 2 compares other demographic 

characteristics of the two groups who reported “Yes” in NHANES. The other demographic characteristics between 

the two groups are similar which again suggests that perhaps the not linked group was receiving CHIP health 

insurance coverage. For those who reported “Yes” in NHANES there were no statistically significant differences 

based on concordance for a participants age or gender, however there were slight differences based on race and 

Hispanic origin (p-value= 0.04). 

Table 2. Demographic characteristics of 1999-2004 NHANES participants aged 0-171 years who reported receiving 

Medicaid/CHIP health insurance coverage by concordance with the CMS MAX database 

Concordant 

“Yes” MAX file| 

“Yes” NHANES 

n=3,101 

Discordant 

“No” MAX file| 

“Yes” NHANES 

n=485 

Characteristic Weighted Percent (SE) Weighted Percent 

Gender: Male 51.7 (1.0) 51.8 (4.1) 

Age (years): <6 41.8 (1.6) 44.9 (3.8) 

Race/ ethnicity 

non-Hispanic white 53.1 (3.9) 48.7 (5.4) 

non-Hispanic black 30.3 (2.9) 26.4 (4.4) 

Mexican American 16.6 (2.6) 25.0 (4.6) 

1 For participants aged 0-16 the responses were answered by a proxy 

For those who did not report Medicaid/CHIP coverage in NHANES but were linked to the CMS MAX database, 

they had a lower (p-value <0.001) mean income to poverty threshold ratio (mean=1.3, standard error=0.05) 

compared to those who did not report Medicaid/CHIP and were not linked (mean= 3.2, standard error=0.06). It is 

possible that those who reported “No” in NHANES but were linked did not know they were enrolled in 

Medicaid/CHIP because they were enrolled in a managed care program through Medicaid. Table 3 compares other 

demographic characteristics of the two groups that reported “No” in NHANES. The characteristics are different by 

age and race and Hispanic origin (p-values <0.001). There were no statistically significant differences by gender. 

The demographic characteristics of the discordant pairs look more similar to those who reported receiving 

Medicaid/CHIP health insurance coverage in terms of age and race and Hispanic origin (see table 2). 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 3. Demographic characteristics of 1999-2004 NHANES participants aged 0-171 years who did not report 

receiving Medicaid/CHIP health insurance coverage by concordance with the Medicaid Max database 

Concordant 

“No” MAX file| 

“No” NHANES 

n=5,057 

Discordant 

“Yes” MAX file| 

“No” NHANES 

n=1,123 

Characteristic Weighted Percent (SE) Weighted Percent 

Gender: Male 50.9 (0.8) 55.5 (2.3) 

Age (years): <6 29.4 (1.0) 36.7 (2.4) 

Race/ ethnicity 

non-Hispanic white 82.0 (1.2) 50.3 (4.8) 

non-Hispanic black 8.9 (0.9) 37.1 (4.9) 

Mexican American 9.1 (1.1) 12.6 (2.2) 

1 For participants aged 0-16 the responses were answered by a proxy 

Conclusions 

There was some potential misreporting in the NHANES interview. However, some differences between the data 

from the two sources may be due to the lack of reporting of CHIP data in the CMS MAX files for those who 

reported “Yes” in NHANES but did not match to the Medicaid file. Our findings for those survey respondents 

reporting no Medicaid/ CHIP but linking to the CMS MAX data are similar to those results reported in the SNACC 

Medicaid undercount project using the Current Population Survey (SNACC is an acronym for the federal agencies 

conducting the project the University of Minnesota's State Health Access Data Assistance Center (SHADAC), 

NCHS, the Agency for Healthcare Research and Quality (AHRQ), ASPE, CMS, and the U.S. Census Bureau). More 

information on the SNACC project can be accessed at http://www.census.gov/did/www/snacc/index.html. Of those 

who reported “No” in NHANES and linked to Medicaid, 44% reported being covered by some other private 

insurance in the NHANES questionnaire and 54% reported they were covered by some other government insurance 

in the NHANES questionnaire. 

In conclusion, 88% (3,101/3,586) of those who reported having Medicaid/CHIP health insurance coverage in 

NHANES linked to the Medicaid enrollment records within 1 month of the NHANES interview. 

References 

Curtin LR, Mohadjer L, Dohrmann S, et al. The National Health and Nutrition Examination Survey: Sample design, 

1999–2006. National Center for Health Statistics. Vital Health Stat 2(155). 2012.