Contribution of Medicaid Managed Care to
the Increasing Undercount of Medicaid
Beneficiaries in the Current Population
Survey
Arpita Chattopadhyay, Ph.D.
&
Andrew B. Bindman, MD
Primary Care Research Center & the Division of
General Internal Medicine
University of California, San Francisco
Background
• CPS is an important source of data for comparing
•
•
•
•
beneficiaries across insurance groups
CPS routinely underestimates the size of Medicaid
beneficiaries.
This affects Medicaid counts, AND estimates of
one or both uninsured/privately insured groups.
The extent of Medicaid undercount has been
increasing over time
At the same time managed care has increased in
Medicaid
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Study Objective
•
To determine if Medicaid managed care is associated
with the undercount of Medicaid beneficiaries in the
CPS.
Study Population
•
•
Medicaid beneficiaries in California < 65 years
Approximately 16% of Medicaid beneficiaries in the
country
Study Period
•
1995-1999: A period of managed care expansion
among Medicaid beneficiaries in California
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Data
• Current Population Survey 1996-2000
– Have you had Medicaid in the past year?
– For how many months did you have Medicaid
last year?
• Medi-Cal Monthly Eligibility Files 19951999
– Monthly enrollment data with information on
county and health plan
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Calculating the Size of the Medicaid
Population
CPS estimate = (S wi * mcaidi*monthsi)/12
wi = individual weight;
mcaidi=1 if the ith respondent ever had Medicaid the
previous year; 0 otherwise;
monthsi= No. of months the ith respondent had Medicaid
the previous year
MMEF count = S No. of beneficiary each month/12
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Method
•
Person-Year estimates of Medicaid beneficiaries for the
years 1995 to 1999 derived from CPS
•
Compared to annual counts (person-years) of Medicaid
beneficiaries from administrative records for the same years
•
County level estimates from CPS derived for 24 of the 58
California counties and a “super county” consisting of the 34
remaining counties
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Variable Definition
Dependent Variable: Percentage of CPS undercount
Yit = 100*(MMEF countit-CPS estimateit)/MMEF countit
i=1….25; t=1…5;
Where ‘i’ represents a county/super-county & ‘t’ represents year
Explanatory variables: Managed care penetration
MCPit = (MMEF managed care countit/ MMEFit)*100
Year
1995-1999
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Model
Yit = a0+a1MCPit+a2YEAR +eit;
i=1….25; t=1…5;
Where a0 is the intercept term; a1 and a2 are
coefficients associated with managed care
penetration and year; eit is the error term
with a variance component correlation
structure
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Medicaid Population Aged < 65 Years Estimated
from Current Population Survey (CPS) and Medicaid
Monthly Eligibility File (MMEF)
Year % undercount % Managed
in CPS
Care
1995
28.98
20.24
1996
32.07
25.53
1997
35.00
37.88
1998
32.17
48.76
1999
33.22
56.23
Source: Current Population Survey 1996-2000 & Medical Care Statistics Section,
Department of Health Services, California
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Unadjusted Association Between Percent Underestimate in
Medicaid Beneficiaries <65 Years from the Current
Population Survey (CPS) and Medicaid Monthly Eligibility
Files (MMEF) 1995-1999
Percent Underestimate
(CPS estimate)
150
100
50
0
-50
-100
-150
Weighted R = 0.23 (p =0.01)
-200
0
20
40
60
80
100
Percent in Managed Care (MMEF count)
Source: Current Population Survey (CPS) 1996-2000 and California Department of Health Services Medicaid
Monthly Eligibility File (MMEF)1995-1999. Percent underestimate corresponds to CPS estimate as
compared to MMEF counts as gold-standard
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Estimated Coefficients for Current Population Survey (CPS)
Underestimate of Medicaid Population <65 Years
Variable
Coefficient1 SE2 P-Value
Year
-2.71
2.43
0.267
Medi-Cal managed
care penetration
Constant
0.40
0.14
0.006
23.92
6.78 <0.001
Source: Current Population Survey 1996-2000 & Medical Care Statistics Section,
Department of Health Services, California
Note: 1 Regressions were weighted by CPS sample size.
2 Robust standard errors
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Implication
• National data show that during 1995-1997
– Medicaid managed care penetration increased by 18.4%
– Underestimate of Medicaid beneficiaries increased by
9%
• Applying our results to national data implies that
– 0.4*18.4= 7.4% increase in the underestimate of
Medicaid beneficiaries in the CPS can be attributed to
increases in managed care penetration
• Thus Medicaid managed care explains more than
80% ( 7.4% of 9.0%) of the increasing
underestimates of Medicaid beneficiaries in the
CPS
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Conclusion
• The CPS underestimated the Medicaid population
by approximately a third during the study period.
• At the county level, each percentage point increase
in the penetration of managed care was associated
with an underestimate in the CPS of 0.4
percentage points.
• Medicaid managed care explains more than 80%
of the increasing undercount of Medicaid
beneficiaries in CPS
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Relevance for Survey Design
• Include question on health plan
characteristics to improve estimates of
Medicaid population in CPS
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Limitations
• Single state study
• Unable to exclude institutional population
from administrative records
• County level estimates from CPS are not
always stable
• Finally, confounding by some other county
characteristic remains a possibility
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