Challenging the norm: further psychometric investigation of the neck disability index

Abstract Background context The neck disability index (NDI) was the first patient-reported outcome (PRO) instrument specific to patients with neck pain, and it remains one of the most widely used PROs for the neck population. The NDI is an appealing measure as it is a short and well-known PRO measure. Currently, there are conflicting data on the performance and applicability of the NDI in patients undergoing either operative or nonoperative treatment for neck-related conditions. Purpose This study investigates the psychometric properties, performance, and applicability of the NDI in the spine patient population. Study design A total of 865 patients visiting a university-based spine clinic with neck complaints, with or without radiating upper extremity pain, numbness, or weakness were enrolled in the study. Visit types included new and follow-up visits to both operative and nonoperative treatments. Questionnaires were administered electronically on a tablet computer, and all patients answered all 10 questions of the NDI. Methods Standard descriptive statistics were performed to describe the demographic characteristics of the patients. Rasch modeling was applied to examine the psychometric properties of the NDI. Results The NDI demonstrated insufficient unidimensionality (ie, unexplained variance after accounting for the first dimension=9.4%). Person reliability was 0.85 and item reliability was 1.00 for the NDI. The overall item fit for the NDI was good with an outfit mean square of 1.03. The NDI had a floor effect of 35.5% and ceiling effect of 4.6%. The raw score to measure correlation of the NDI was 0.019. Conclusions Although the NDI had good person and item reliability, it did not demonstrate strong evidence of unidimensionality. The NDI exhibited a very large floor effect. Because of the poor raw score to measure correlation, the sum score should not be used in interpretation of findings. Despite great investment by physicians and other stakeholders in the NDI, this evaluation and previous research have demonstrated that the NDI needs further investigation and refinement.