Definition of the construct to be measured is a prerequisite for the assessment of validity. The Neck Disability Index as an example.

Objective: To determine the content, structural, and construct validity of the Dutch version of the Neck Disability Index (NDI). Study Design and Setting: To assess content validity, 11 neck pain experts and 10 patients commented on the construct, comprehensiveness, and relevance of the NDI. Structural validity was assessed by item factor analysis (FA) and item response theory modeling using the generalized partial credit model. Differential item functioning (DIF) analysis for gender was examined. Pearson correlation coefficient with the Disabilities of the Arm, Shoulder and Hand (DASH) questionnaire was calculated to assess construct validity. Results: In addition to a suboptimal translation, we found a lack of consensus on the construct the NDI intends to measure. Experts and patients suggested that the NDI measures more than physical functioning. Unidimensionality of the NDI could not be confirmed. DIF analysis for gender showed DIF for the headache item. The goodness-of-fit statistics for FA with one factor were satisfactory when the item "concentration" was omitted. A correlation of 0.75 with the DASH was found supporting construct validity. Conclusion: It is questionable whether in research the NDI should be the instrument of choice for use as a primary outcome measure. Definition of the construct to be measured is a prerequisite for the assessment of validity. © 2013 Published by Elsevier Inc.