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