Rasch analysis of the intermittent and constant osteoarthritis pain (ICOAP) scale

OBJECTIVES: The Intermittent and Constant Osteoarthritis Pain (ICOAP) questionnaire was developed to assess two forms of pain reported by people with osteoarthritis: intermittent and constant pain. Studies examining its measurement qualities have provided some support for its use as separate and total scales. However, it has not been previously evaluated using Rasch analysis. The current study examined the fit between data obtained from the ICOAP questionnaire and the Rasch model to determine whether it meets the requirements of interval-level measurement. DESIGN: ICOAP responses from 175 participants with knee osteoarthritis were collected in a cross-sectional questionnaire study. Participants were recruited from hospital clinics and a group who had taken part in previous research. The questionnaires were completed at home and returned by pre-paid envelope and the data were analysed using RUMM2020. RESULTS: Fit to the Rasch model was achieved for both the Constant and Intermittent subscales following removal of a small number of items. The Total scale initially resulted in substantial misfit to the model, but fit was improved by removing four items that misfit the model. However, several participants presented with high fit residuals, which is consistent with misfit. CONCLUSIONS: The results support the use of Constant and Intermittent subscales as unidimensional measures of pain. The Total scale can be adapted to improve fit to the Rasch model, but there are concerns over participant misfit.