An application of Rasch analysis to the measurement of communicative functioning

The purposes of this investigation were to examine the construct dimensionality and range of ability effectively measured by 28 assessment items obtained from 3 different patient-reported scales of communicative functioning, and to provide a demonstration of how the Rasch approach to measurement may contribute to the definition of latent constructs and the development of instruments to measure them.

Item responses obtained from 421 stroke survivors with and without communication disorders were examined using the Rasch partial credit model. The dimensionality of the item pool was evaluated by (a) examining correlations of Rasch person ability scores obtained separately from each of the 3 scales, (b) iteratively excluding items exceeding mean square model fit criteria, and (c) using principal-components analysis of Rasch model residuals. The range of ability effectively measured by the item pool was examined by comparing item difficulty and category threshold calibrations to the distribution of person ability scores and by plotting the modeled standard error of person ability estimates as a function of person ability level.

The results indicate that most assessment items fit a unidimensional measurement model, with the notable exception of items relating to the use of written communication. The results also suggest that the range of ability that could be reliably measured by the current item pool was restricted relative to the range of ability observed in the patient sample.

It is concluded that (a) a mature understanding of communicative functioning as a measurement construct will require further research, (b) patients with stroke-related communication disorders will be better served by the development of instruments measuring a wider range of communicative functioning ability, and (c) the theoretical and methodological tools provided by the Rasch family of measurement models may be productively applied to these efforts.