Nursing Home Residence Confounds Gender Differences in Medicare Utilization: An Example of Simpson's Paradox
Received 26 December 2007; received in revised form 1 August 2009; accepted 21 November 2009. published online 12 February 2010.
Background
Gender differences in health care utilization in older Americans may be confounded by nursing home residence. Medicare data contain several files that can be used to create a measure of nursing home residence, but prior work has not addressed which best account for potential confounding. Simpson's paradox occurs when aggregated data support a different conclusion from what the disaggregated data show. We describe such a paradox that appeared when we sharpened our definition of “nursing home residence” while examining gender differences in Medicare utilization at the end of life.
Methods
To understand gender-specific health care utilization at the end of life, we conducted a retrospective analysis of a national random sample of Medicare beneficiaries aged 66 or older who died in 2001 with Parts A and B data for 18 months before death. We sought to associate each of total hospital days and costs during the final 6 months of life with numbers of primary care physician visits in the 12 preceding months. In addition to demographics, comorbidities, and geography, “nursing home residence” was a potential confounder, which we imputed in two ways: 1) from skilled nursing facility bills in the Part A Medicare Provider Analysis and Review (MedPAR) file; and 2) from Berenson-Eggers-Type-of-Service codes indicating widely spaced doctor visits in nursing homes obtained from Medicare's carrier file.
Conclusion
Gender differences in Medicare utilization are strongly confounded by nursing home resident status, which can be imputed well from Medicare's carrier file, but not MedPAR.
aWomen's Health and Health Care Research Units, Section of General Internal Medicine, Evans Department of Medicine, Boston University Medical Center, Boston, Massachusetts
bDepartment of Clinical Bioethics, Warren Magnuson Clinical Center, National Institutes of Health, Bethesda, Maryland
cWomen's Health Interdisciplinary Research Center, Boston University School of Medicine, Boston, Massachusetts
Correspondence to: Andrea C. Kronman, MD, MSc, Women's Health Unit, 801 Massachusetts Avenue, Suite 470, Boston, MA 02118; Phone: 617-638-8036; Fax: 617-638-8096.
Funded by the Office of Research on Women's Health (ORWH) K12-43444, with support from the Centers for Medicare and Medicaid Services (CMS) and the National Cancer Institute (NCI).
ABSTRACT