Measurement of change in health status with Rasch models

Background

The traditional approach to the measurement of change presents important drawbacks (no information at individual level, ordinal scores, variance of the measurement instrument across time points), which Rasch models overcome. The article aims to illustrate the features of the measurement of change with Rasch models.

Methods

To illustrate the measurement of change using Rasch models, the quantitative data of a longitudinal study of heart-surgery patients (N = 98) were used. The scale “Perception of Positive Change” was used as an example of measurement instrument. All patients underwent cardiac rehabilitation, individual psychological intervention, and educational intervention. Nineteen patients also attended progressive muscle relaxation group trainings. The scale was administered before and after the interventions. Three Rasch approaches were used. Two separate analyses were run on the data from the two time points to test the invariance of the instrument. An analysis was run on the stacked data from both time points to measure change in a common frame of reference. Results of the latter analysis were compared with those of an analysis that removed the influence of local dependency on patient measures. Statistics t, χ² and F were used for comparing the patient and item measures estimated in the Rasch analyses (a-priori α = .05). Infit, Outfit, R and item Strata were used for investigating Rasch model fit, reliability, and validity of the instrument.

Results

Data of all 98 patients were included in the analyses. The instrument was reliable, valid, and substantively unidimensional (Infit, Outfit < 2 for all items, R = .84, item Strata range = 3.93-6.07). Changes in the functioning of the instrument occurred across the two time, which prevented the use of the two separate analyses to unambiguously measure change. Local dependency had a negligible effect on patient measures (p ≥ .8674). Thirteen patients improved, whereas 3 worsened. The patients who attended the relaxation group trainings did not report greater improvement than those who did not (p = .1007).

Conclusions

Rasch models represent a valid framework for the measurement of change and a useful complement to traditional approaches.

Evaluation of the MOS SF-36 physical functioning scale (PF-10): I. Unidimensionality and reproducibility of the Rasch item scale

Indexes developed to measure physical functioning as an essential component of general health status are often based on sets of hierarchically-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 item positions (calibrations) along a scale. Data gathered on the 10-item Physical Functioning Scale (PF-10) from a large sample of Medical Outcomes Study patients (N = 3445) were used to examine the hierarchical order, unidimensionality, and reproducibility of item calibrations. Rasch-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 repeated 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 Likert summative scores. Unidimensionality and reproducibility of the item scale are essential prerequisites for the development of Rasch-based person measures of physical functioning that can be used across populations and over repeated tests.