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

2 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

3 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 : A period of managed care expansion among Medicaid beneficiaries in California

4 Data Current Population Survey –Have you had Medicaid in the past year? –For how many months did you have Medicaid last year? Medi-Cal Monthly Eligibility Files –Monthly enrollment data with information on county and health plan

5 Calculating the Size of the Medicaid Population CPS estimate = ( w i * mcaid i *months i )/12 w i = individual weight; mcaid i =1 if the i th respondent ever had Medicaid the previous year; 0 otherwise; months i = No. of months the i th respondent had Medicaid the previous year MMEF count = No. of beneficiary each month/12

6 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

7 Variable Definition Dependent Variable: Percentage of CPS undercount Y it = 100*(MMEF count it -CPS estimate it )/MMEF count it i=1….25; t=1…5; Where i represents a county/super-county & t represents year Explanatory variables: Managed care penetration MCP it = (MMEF managed care count it / MMEF it )*100 Year

8 Model Y it = a 0 +a 1 MCP it +a 2 YEAR +e it ; i=1….25; t=1…5; Where a 0 is the intercept term; a 1 and a 2 are coefficients associated with managed care penetration and year; e it is the error term with a variance component correlation structure

9 Medicaid Population Aged < 65 Years Estimated from Current Population Survey (CPS) and Medicaid Monthly Eligibility File (MMEF) Year% undercount in CPS % Managed Care Source: Current Population Survey & Medical Care Statistics Section, Department of Health Services, California

10 Unadjusted Association Between Percent Underestimate in Medicaid Beneficiaries <65 Years from the Current Population Survey (CPS) and Medicaid Monthly Eligibility Files (MMEF) Source: Current Population Survey (CPS) and California Department of Health Services Medicaid Monthly Eligibility File (MMEF) Percent underestimate corresponds to CPS estimate as compared to MMEF counts as gold-standard

11 Estimated Coefficients for Current Population Survey (CPS) Underestimate of Medicaid Population <65 Years VariableCoefficient 1 SE 2 P-Value Year Medi-Cal managed care penetration Constant <0.001 Source: Current Population Survey & Medical Care Statistics Section, Department of Health Services, California Note: 1 Regressions were weighted by CPS sample size. 2 Robust standard errors

12 Implication National data show that during –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

13 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

14 Relevance for Survey Design Include question on health plan characteristics to improve estimates of Medicaid population in CPS

15 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