title: Medigap premiums and Medicare HMO enrollment.
abstract: OBJECTIVE
  Markets for Medicare HMOs (health maintenance organizations) and supplemental Medicare coverage are often treated separately in existing literature. Yet because managed care plans and Medigap plans both cover services not covered by basic Medicare, these markets are clearly interrelated. We examine the extent to which Medigap premiums affect the likelihood of the elderly joining managed care plans.
  
  
  METHODS
  The analysis is based on a sample of Medicare beneficiaries drawn from the 1996-1997 Community Tracking Study (CTS) Household Survey by the Center for Studying Health System Change. Respondents span 56 different CTS sites from 30 different states. Measures of premiums for privately-purchased Medigap policies were collected from a survey of large insurers serving this market. Data for individual, market, and HMO characteristics were collected from the CTS, InterStudy, and HCFA (Health Care Financing Administration).
  
  
  METHODS
  Our analysis uses a reduced-form logit model to estimate the probability of Medicare HMO participation as a function of Medigap premiums controlling for other market- and individual-level characteristics. The logit coefficients were then used to simulate changes in Medicare participation in response to changes in Medigap premiums.
  
  
  RESULTS
  We found that Medigap premiums vary considerably among the geographic markets included in our sample. Measures of premiums from different insurers and for different types of Medigap policies were generally highly correlated across markets. Our models consistently indicate a strong positive relationship between Medigap premiums and HMO participation. This result is robust across several specifications. Simulations suggest that a one standard deviation increase in Medigap premiums would increase HMO participation by more than 8 percentage points.
  
  
  CONCLUSIONS
  This research provides strong evidence that Medigap premiums have a significant effect on seniors' participation in Medicare HMOs. Policy initiatives aimed at lowering Medigap premiums will likely discourage enrollment in Medicare HMOs, holding other factors constant. Although the Medigap premiums are just one factor affecting the future penetration rate of Medicare HMOs, they are an important driver of HMO enrollment and should be considered carefully when creating policy related to seniors' supplemental coverage. Similarly, our results imply that reforms to the Medicare HMO market would influence the demand for Medigap policies.
authors.affiliation: Department of Health Management & Policy, School of Public Health, The University of Michigan, Ann Arbor 48109-2029, USA.
authors.forename: Catherine G
authors.initials: CG
authors.lastname: McLaughlin
authors.forename: Michael
authors.initials: M
authors.lastname: Chernew
authors.forename: Erin Fries
authors.initials: EF
authors.lastname: Taylor
mesh_terms: D000368:Aged; D003657:Decision Making; D005249:Fees and Charges; D005260:Female; D006279:Health Maintenance Organizations / Q000191:economics* / Q000706:statistics & numerical data*; D006302:Health Services Research; D006801:Humans; D019458:Insurance Coverage; D017054:Insurance, Medigap / Q000191:economics*; D008297:Male; D020398:Medicare Part C / Q000191:economics*; D017059:Models, Econometric; D011336:Probability