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The Outcome and Assessment Information Set (OASIS) is the universal clinical assessment tool for adult nonmaternity patients receiving skilled care at home from Medicare-certified home health agencies in the United States. Anticipating increased use of OASIS data for research purposes, this article explored the usefulness of Rasch modeling to address disability measurement challenges presented by the unique response category structure of the seven activities of daily living (ADL) and eight instrumental ADL (IADL) items in the OASIS.To illustrate how Rasch model statistics can be used to evaluate OASIS ADL and IADL item unidimensionality and model fit; to illustrate how Rasch modeling simultaneously estimates ADL and IADL item difficulty, thresholds between item response categories, and person disability; and to compare Rasch estimates of item difficulty and person disability scores to estimates based on more conventional Likert scoring techniques.Medicare-eligible home health care patients (n = 583) served by one of 12 home care agencies in Ohio between November 1999 and September 2000.ADL and IADL items were measured three ways: according to the original OASIS scoring (raw Likert); transformed raw Likert scores accounting for the nonuniform item structure (corrected Likert); and Rasch Partial Credit model scores.The items bathing and telephone use showed evidence of unexpected response patterns; recoding of these items was necessary for good Rasch model fit. Partial Credit model results revealed that interval distances between response categories varied widely across the 15 ADL and IADL items. When ADL and IADL items were ranked by level of difficulty, results were similar between Rasch and corrected Likert measurement approaches; however, corrected Likert person scores were found to be nonlinear at highest and lowest disability levels when plotted against Rasch person scores.Rasch modeling can help improve the precision of disability measurement in Medicare home care patients when using ADL and IADL items from the OASIS instrument.