Hierarchical nature of activities of daily living in the Spanish Disability Survey.

Many studies have addressed the issue of dimensionality in activities of daily living (ADL) and its hierarchy, with contradictory results. This paper aims to study the unidimensional structure and the hierarchy of ADL in the Spanish Disability Survey. We analysed a sample of 8381 people aged 65 and over with at least one disability and receiving personal help with one of them. The following Rasch model properties were studied in 14 items enquiring about basic and instrumental ADL: fit to the model, reliability, unidimensionality, local independency between items and differential item functioning (DIF) by gender, age and proxy. The unidimensionality of the scale was confirmed. The final analysis of eight items showed a satisfactory fit, good reliability, local independency, unidimensionality and no DIF by age. The disability linear measure showed significant differences by gender and age. The obtained disability scale is a simple and reliable measure, and it suggests a hierarchical order of ADL and predicts a schedule of functional impairment in older adults. Rating the disability of older community-dwelling people has a predictive value that policy makers might find useful in service planning.