tailieunhanh - The Effect of HMOs on the Inpatient Utilization of Medicare Beneficiaries Technical Appendix

The before and after with the always in FFS comparison group aspect of the data is captured using cohort indicator variables that represent switching to HMO and staying or always remaining in FFS, which average the behavior and other unobserved characteristics of individuals in these cohorts. | WORKING P A P E R The Effect of HMOs on the Inpatient Utilization of Medicare Beneficiaries Technical Appendix NASREEN DHANANI JUNE F. O LEARY EMMETT KEELER ANIL BAMEZAI GLEnN MELNICK WR-138 February 2004 This Working Paper is the technical appendix to an article published in a scientific journal. It has been subject to the journal s usual peer review process. RAND is a registered trademark. RAND HEALTH TECHNICAL APPENDIX The Effect of HMOs on the Inpatient Utilization of Medicare Beneficiaries. In this Appendix we explain and justify our analytic approach to estimating the pure managed care effect of Medicare HMOs. We used a before and after with a comparison group design to evaluate the impact of Medicare HMOs on hospital To take advantage of our enormous data set with up to five years of inpatient data 1991-1995 on millions of Medicare beneficiaries in which the HMO people spent considerable time in and out of an HMO we were somewhat restrictive in selecting a clean HMO sample. See Study Design Overview and Sample Selection portions of paper . After estimating the reduction in utilization due to being in an HMO in our selected sample we assume that this managed care effect is proportionally the same for everyone both in and out of this sample. We then calculate the selection effect indirectly as the difference between FFS use and what we hypothesize the people in HMOs would use if they were in FFS. Other assumptions and choices made The unit of analysis is the person year. We descriptively evaluated utilization and deaths by quarter and even by month in smaller samples and found no cyclical patterns or shortrun differences in these variables around the time of changes from FFS to HMO or changes from HMO to FFS. Therefore we defined the in HMO variable for each year by the ratio of the months people are in an HMO over the months that people are alive in that year. Using the person year in a two-part model means that the first part is whether a .