EVALUATION OF THE MOS SF-36 PHYSICAL FUNCTIONING SCALE (PF-10) .1. UNIDIMENSIONALITY AND REPRODUCIBILITY OF THE RASCH ITEM SCALE

Indexes developed to measure physical functioning as an essential comp onent of general health status are often based on sets of hierarchical ly-structured items intended to represent a broad underlying concept. Rasch Item Response Theory (IRT) provides a methodology to examine the hierarchical structure, unidimensionality, and reproducibility of ite m positions (calibrations) along a scale. Data gathered on the 10-item Physical Functioning Scale (PF-10) from a large sample of Medical Out comes Study patients (N = 3445) were used to examine the hierarchical order, unidimensionality, and reproducibility of item calibrations. Ra sch-IRT analyses generated an empirical item hierarchy, confirmed the unidimensionality of the PF-10 for most patients, and established the reproducibility of item calibrations across patient populations and re peated tests. These findings support the content validity of the PF-10 as a measure of physical functioning and suggest that valid Rasch-IRT summary scores could be generated as an alternative to the current Li kert summative scores. Unidimensionality and reproducibility of the it em scale are essential prerequisites for the development of Rasch-base d person measures of physical functioning that can be used across popu lations and over repeated tests.