Chapter 5. HMO Weight adjustment

Claim data may be incomplete or missing for Medicare beneficiaries enrolled in a Medicare Advantage plan or Health Maintenance Organization (HMO) because payment of encounters with the health care system are paid by the private insurer, not CMS. Across the years of study, 1999-2009, between 20-30% of MCBS participants were enrolled in an HMO each year. We performed weight adjustments to account for incomplete claim information for each of the community and institutionalized MCBS participants enrolled in an HMO each year. Weights were calculated to adjust for participants not considered as "pure" Medicare enrollees. "Pure" was defined as follows: 1. no participation in an HMO for the year of study, and 2. enrollment in Medicare parts A & B for the full 12-month study period unless the participant died during the year.

To calculate the adjusted weights, logistic regression modeling was conducted to model "pure" Medicare participation using select covariates including any combination of demographic, SES, health status, and ADL/IADls variables and interaction terms to achieve decent model fit. Covariates included in the models vary across years and may vary across calibrated sets. Appendix 5a contains covariates included in each model. Model fit was assessed by Hosmer-Lemeshow test. Using the predicted probability (p) of "pure", the "pure MC" weight was calculated as 1/p. To assess balance in the community dwelling population, propensity of pure was estimated using GLM; F-ratios were reviewed for significance. In the institutionalized population, regression models were performed for each covariate to assess the association with propensity for pure Medicare participation. Using the residuals from each model, we calculated effect size to assess balance. For both the community and institutionalized populations, we calculated the "final weight" as the product of the existing MCBS survey weight and the "pure MC" weight.

To assess the accuracy of the weight adjustment, we compared the population totals from the 2000 Census, aged 65+ to those represented in the year 2000 analytic files for those 65+ using the weight variable provided in the MCBS data files for those with any Medicare participation and the "final weight" for those with "pure" participation. The population size differed from the Census by about 5.5% for the 65+ Medicare population and by about 6.5% for those 65+ with "pure" enrollment.

The final analytic file used for the related analyses was restricted to participants considered to have "pure" Medicare enrollment. To account for participants with incomplete claim data due to participation in an HMO and the population restriction we followed, the "final weight" was used for all analyses.