Validity of the Neck Disability Index and Neck Pain and Disability Scale for measuring disability associated with chronic, non-traumatic neck pain.

The purpose of this study was to evaluate the construct and content validity of the Neck Disability Index (NDI) and the Neck Pain and Disability Scale (NPAD) in patients with chronic, non-traumatic neck pain. Twenty patients (mean age=64.5 years) completed a patient-specific questionnaire, the Problem Elicitation Technique (PET), followed by the NDI and NPAD. Content validity was assessed by comparing the items of the NDI and NPAD with problems identified from the PET. Construct validity of the fixed-item questionnaires was examined by establishing the correlation with each other, and with the PET score. Eleven common problems were identified by patients through the PET, of which six were included in the NDI and seven included in the NPAD. The NDI and NPAD scores were strongly correlated (r=0.86, p<0.01), while the correlation between the PET and the fixed-item questionnaires was moderate (NDI: r=0.62, p<0.01; NPAD: r=0.71, p<0.01). Both the NDI and the NPAD include most of the functional problems common to this patient group, and display good content validity. The PET is better able to evaluate the problems specific to the individual patient and is therefore measuring a somewhat different construct to the fixed-item questionnaires.