Rasch analysis of functional assessment scales: an example using pain behaviors

Functional assessments associated with rehabilitation frequently rely on scoring systems based upon a limited set of categorical values, arranged in order and intended to represent an underlying performance continuum. Making sense of data generated by these scales is a challenge well met by the Rasch measurement model, which allows detailed investigation of how individuals are likely to perform on a given occasion and indicates whether changes in performance occur in orderly patterns over repeated occasions. Additionally, the Rasch measurement model allows study of the quality of measurement and the degree to which the scoring adequately reflects the underlying, but not directly measurable, continuum. Data gathered at admission and discharge from 330 patients attending a rehabilitation program for chronic low back pain were evaluated using the Rasch measurement model, and the results were contrasted with those from the best available nonparametric technique, ridit analysis.