Item response theory assumptions were adequately met by the Oxford Hip and Knee Scores.

OBJECTIVES: To develop item response theory (IRT) models for the Oxford Hip and Knee Scores (OHS/OKS) which convert patient responses into continuous scores with quantifiable precision, and provide these as web applications for efficient score conversion. STUDY DESIGN AND SETTING: Data from the NHS PROMs programme were used to test the assumptions of IRT (unidimensionality, monotonicity, local independence, and measurement invariance) before fitting models to preoperative response patterns obtained from patients undergoing primary elective hip or knee arthroplasty. The hip and knee datasets contained 321,147 and 355,249 patients, respectively. RESULTS: Scree plots, Kaiser criterion analyses, and confirmatory factor analyses confirmed unidimensionality and Mokken analysis confirmed monotonicity of both scales. In each scale, all item pairs shared a residual correlation of ≤ 0.20. At the test-level, both scales showed measurement invariance by age and gender. Both scales provide precise measurement in preoperative settings, but demonstrate poorer precision and ceiling effects in postoperative settings. CONCLUSIONS: We provide IRT parameters and web applications that can convert OHS or OKS response sets into continuous measurements and quantify individual measurement error. These can be used in sensitivity analyses or to administer truncated and individualized computerized adaptive tests.