Assessment of perceived functional capacity: Using Rasch analysis to evaluate the measurement properties of four perceived pain & disability scales

Functional Capacity Evaluations (FCEs) include comparisons of self-report and performance-based measures. A difference in the two scores can be interpreted as symptom magnification which can impact eligibility for benefits. FCEs typically include scales such as the Oswestry Disability Index (ODI), the Dallas Pain Questionnaire (DPQ), the Spinal Function Sort (SFS) and the Neck Disability Index (NDI). Rasch Modeling was used to evaluate their original classification categories. Examination included fit of data to model expectations, threshold ordering of items, differential item functioning and item difficulty. None of the scales demonstrated unidimensionality. For the ODI and DPQ, rescaling and/or eliminating items improved the scales. The SFS is not a unidimensional scale and demonstrates differential item functioning. The NDI demonstrates unidimensionality when two of the items are eliminated but disordered thresholds could not be fixed. Health Professionals using these measures should be aware that these scales do not perform as well as expected.