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Measures of functional disability typically contain items that reflect limitations in performing activities of daily living (ADLs) or instrumental activities of daily living (IADLs). Combining IADL and ADL items together in the same scale would provide enhanced range and sensitivity of measurement. This article presents psychometric justification for a combined ADL/IADL scale. Data come from 2,977 disabled respondents in the 1989 National Long-Term Care Survey. Respondents indicated whether they received human help on 7 ADL items; they also indicated whether they were unable to perform each of 9 IADL items due to health reasons. Factor analyses using tetrachoric correlations demonstrated that 15 of the 16 items reflected one major dimension. Item response theory (IRT) methods were used to calibrate the items; a one-parameter IRT model fit the data. Item calibrations showed that ADL and IADL items were not hierarchically related. Analyses showed that a simple sum of item responses could be used to derive a measure of functional disability. Implications of using a 15-item ADL/IADL scale for eligibility determination and for comparing groups are discussed.
Spector et al. (Thu,) studied this question.