The Medicaid Undercount in the 

Current Population Survey 

James Noon 

U.S. Census Bureau 

Federal Committee on Statistical Methodology 

Research Conference 

November 4, 2013 
Overview of Presentation Topics

• Background 

• Estimating the undercount in the Current 

Population Survey Annual Social and 

Economic Supplement (CPS ASEC) 

• Response error in the CPS ASEC 

• Conclusions 

2 
Background

• Health insurance data collected by the CPS ASEC is 

used to estimate the number, percentage and 

characteristics of the uninsured population. 

• CPS ASEC estimates of Medicaid enrollment are 

consistently lower than the enrollment rates reported in 

the Medicaid Statistical Information System (MSIS) 

database. 

3 
Background

• The SNACC project and the Medicaid undercount 

• The SNACC project collaborators included: 

– The University of Minnesota's State Health Access Data 

Assistance Center (SHADAC) 

– The National Center for Health Statistics (NCHS) 

– The Agency for Healthcare Research and Quality (AHRQ) 

– The U.S. Department of Health and Human Services 

Assistant Secretary for Planning and Evaluation (ASPE) 

– The Centers for Medicare and Medicaid Services (CMS) 

– The U.S. Census Bureau 

• Our research expands analysis of the undercount to CY 2006-

2008.

4 
Data and Methods

• CPS ASEC 2007-2009

• MSIS 2006-2008

• Medicaid undercount 

– Enrollment totals in CPS ASEC universe 

compared to MSIS universe 

• Response error 

– Linked MSIS-CPS ASEC data 

5 
Differences in MSIS and CPS ASEC 

Universes

MSIS CPS ASEC 

Data reported in fiscal quarters Data reported for previous calendar 

year 

Data reported by case Data reported by person 

Data includes institutionalized 

population 

Data does not include institutionalized 

population 

Benefits may be full or restricted to 

particular types of services 

Data on level of benefits (full or 

restricted) not collected 

MSIS data is point-in-time CPS ASEC data retrospective but may 

be reported as point-in-time 

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Undercount Methodologies

• Raw measures: 

– Cases with full or partial benefits 

– Medicaid participants residing in institutional group 

quarters 

– Duplicates arising from individuals having multiple 

accounts 

• Adjusted measures: 

– Limited to unduplicated 

– Non-institutionalized participants 

– Receiving full Medicaid benefits 

7 
Medicaid Undercount in CPS ASEC, 2000-2008

8 

Sources: SNACC (2010) for years 2000-2005; authors’ computations for years 2006-2008; CPS ASEC and MSIS.

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

CY 2000 CY 2001 CY 2002 CY 2003 CY 2004 CY 2005 CY 2006 CY 2007 CY 2008 

34% 35%

38% 37% 37% 38% 39%

37%

35%

32% 32%

38%

32%

31% 32% 31%

29%

26%

Raw measure 

Adjusted measure 
Medicaid Coverage in 

MSIS and CPS ASEC False Negative Error 

False Positive Error No Medicaid Coverage 

CPS ASEC 

MSIS 

 Yes No 

Yes 

No

Measuring Response Error in the 

CPS ASEC

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Percentage Reporting False Negative 

Errors, CY 2000-2008

10

10%

20%

30%

40%

50%

60%

CY 2000 CY 2001 CY 2002 CY 2003 CY 2004 CY 2005 CY 2006 CY 2007 CY 2008 

43% 43%

45% 43% 44% 43%

45% 44%

41%

Sources: SNACC (2010) for years 2000-2005; authors’ computations for years 2006-2008; CPS ASEC and MSIS.
False Negative Errors

• False negative errors are the largest contributor to the 

undercount in CPS ASEC. 

• About 80 percent of these false negative errors come 

from explicitly reported insurance status in the CPS 

ASEC, while the rest result from data imputation 

procedures. 

• In the false negative population, about 60 percent 

report having some other form of insurance. 

11
False Negative Errors by Percent Federal 

Poverty Level (FPL), CY 2008

0 - 49%

50 - 99%

100 - 149%

150 - 199%

200% + 

00% 10% 20% 30% 40% 50% 60%

29%

30%

38%

49%

56%

Percent poverty (FPL) 

Percent reporting no Medicaid coverage and linked to MSIS 

(only explicitly reported enrollment status included) 

Sources: Authors’ computations, CPS ASEC and MSIS 2008. 

12
False Negative Errors by Length of 

Enrollment in Medicaid, CY 2008

<61 days 

61 - 180 days 

>180 days 

00% 10% 20% 30% 40% 50% 60% 70% 80%

75%

55%

35%

Days enrolled in Medicaid 

Percent reporting no Medicaid coverage and linked to MSIS 

(only explicitly reported enrollment status included) 

Sources: Authors’ computations, CPS ASEC and MSIS 2008. 

13
False Negative Errors by Enrollment in Survey 

Year (2009), CY 2008

yes 

no

00% 10% 20% 30% 40% 50% 60% 70%

33%

59%

Enrolled in survey year? 

Percent reporting no Medicaid coverage and linked to MSIS 

(only explicitly reported enrollment status included) 

Sources: Authors’ computations, CPS ASEC and MSIS 2008. 

14
Percentage Reporting False Positive 

Errors, CY 2000-2008

10%

20%

30%

40%

50%

60%

CY 2000 CY 2001 CY 2002 CY 2003 CY 2004 CY 2005 CY 2006 CY 2007 CY 2008 

25%

23% 22% 21%

23% 23%

26% 25% 24%

Sources: SNACC (2010) for years 2000-2005; authors’ computations for years 2006-2008; CPS ASEC and MSIS.

15
False Positive Errors by Percent Federal 

Poverty Level (FPL), CY 2008

0 - 49%

50 - 99%

100 - 149%

150 - 199%

200% + 

0% 2% 4% 6% 8% 10%

7%

7%

5%

3%

1%

Percent poverty (FPL) 

Percent reporting Medicaid coverage and not linked to MSIS 

(only explicitly reported status included) 

Sources: Authors’ computations, CPS ASEC and MSIS 2008. 

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Conclusions

• The CPS ASEC undercount has declined slightly for CY 

2006-2008. 

• False negative and false positive errors have remained 

stable in CY 2000-2008. 

• Poverty is associated with both types of errors in reporting 

of Medicaid coverage. 

• Response error in the CPS ASEC impacts estimates of the 

uninsured population. 

• As Medicaid enrollment expands with the Patient 

Protection and Affordable Care Act, understanding the 

Medicaid undercount in surveys is increasingly important 

for better assessing estimates of the uninsured 

population. 

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Thank You!

james.noon@census.gov

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