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Background: The Barthel Index (BI) is a frequently used measure of independence in the activities of daily living (ADLs). Item functioning of various versions of the BI have been examined using Rasch analysis. Item response theory (IRT) models for ordered polytomous responses may provide more insight into item functioning across levels of independence in ADLs.
Objectives: To compare the fit and appropriateness of the one-parameter logistic model (1PL), the partial credit model (PCM), and the extension of the generalized partial credit model implemented in ConQuest (GPCM-CQ) for the 15-item BI.
Methods: This article is a secondary analysis of baseline BI data obtained from four randomized controlled trials for 788 residents from multiple long-term care facilities. Parameters of three different IRT-based models (1PL Rasch model, PCM, and GPCM-CQ) were estimated. Fit of items and response vectors was assessed. Overall fit was compared across the three models.
Results: Item difficulties were similar for all three models. Most of the 15 items were located at a moderate level of functional independence. In all three models, “don brace” was the easiest ADL but had poor discrimination; “climbing stairs” was the most difficult ADL. Multiple items showed misfit in both 1PL and PCM. Item parameters and person proficiency estimates were highly correlated for the PCM and GPCM-CQ models. The difference in deviance between the PCM and GPCM-CQ was significant. In the GPCM-CQ, most items showed good discrimination, but several had negative or very low discrimination parameters.
Discussion: GPCM-CQ results suggested that further revision of the BI may be warranted. Because some items showed poor discrimination, caution should be used when measuring ADL independence with the BI item set.
