Internal construct validity of the Oxford Knee Scale: Evidence from Rasch measurement

Objective. Symptomatic knee osteoarthritis (OA) is present in 1 in 8 patients age > 60 years and is associated with significant activity limitation. Several tools have been devised to assess knee problems. The goal of this study was to evaluate the Oxford Knee Scale (OKS) against strict modern psychometric standards through application of the Rasch measurement model. Methods. A total of 224 OKS assessments were included from patients with a clinical diagnosis of knee OA. Data from the OKS were fitted to the Rasch measurement model. We examined the validity of the item scoring functions, the presence of item bias or differential item functioning, the fit of data to model expectations, and whether or not the item set formed a unidimensional scale, thus giving a valid summed score. Results. The mean age of the 224 patients was 61 years (range 26-90) and 61.5% were women. After rescoring some items, the scale showed good fit to the Rasch model, with a chi-square interaction statistic of 42.663 (36 df, P = 0.206). Overall targeting of the scale (to the patients) was good, with high reliability. Conclusion. Data from the OKS were consistent with the expectations of the unrestricted (partial credit) derivation of the Rasch model. The targeting of the instrument shows good coverage of thresholds across the whole construct and has good reliability (internal consistency) with a high patient separation. Consequently, this scale can be used in confidence with the knowledge that it is a unidimensional scale largely free of bias.