Original ArticleDefinition of the construct to be measured is a prerequisite for the assessment of validity. The Neck Disability Index as an example
Introduction
A recent systematic review on the measurement properties of disease-specific questionnaires in patients with neck pain indicated that the Neck Disability Index (NDI) is the most frequently evaluated questionnaire [1]. Researchers and clinicians often use the NDI to measure the level of disability and to study the effect of an intervention on patients with neck pain [2], [3], [4], [5]. In developing the NDI, Vernon and Mior were inspired by Fairbank and Pynsent [6] who developed the Oswestry Disability Index (ODI) for measuring disability in patients with low back pain. The NDI was the first instrument designed to assess self-rated disability in patients with neck pain. Currently, there are more than 450 articles internationally that have cited the NDI. Numerous clinical guideline organizations, especially for whiplash management, have endorsed the NDI as the questionnaire of choice for patients with neck pain.
Vernon declared in an e-mail conversation (November 2011) that the construct of the NDI was “self-rated disability,” where disability was understood as the perceived effect of pain and impairment on the patient's performance and enjoyment of activities of daily living.
However, different opinions exist with regard to the meaning of the construct that the NDI aims to measure. Some researchers interpret the NDI as a measure of functional status [7], [8], whereas others have a broader interpretation and see it as a measure of pain and disability [9]. As a result, confusion might arise as to what the NDI aims to measure and how scores should be interpreted.
The lack of consensus in the construct that the NDI aims to measure might hinder an assessment of the validity of the instrument as validity is defined as the extent to which an instrument measures what it purports to measure [10].
The aim of this article was to evaluate the quality of the translation of the most commonly used Dutch version of the NDI (NDI-DV) and to determine its content, structural, and construct validity.
Section snippets
The NDI
The NDI is a patient-reported outcome (PRO) measure. It consists of 10 items. The 10 items have six response categories (range, 0–5; total score range, 0–50) [11] (Appendix A; available on the journal's website at www.jclinepi.com).
Translation
To check the Dutch translation of the NDI, the most commonly used NDI-DV [12] was translated back into English by two independently operating professional translators, blinded to the original English version. Discrepancies between the translated version, the versions
Quality of the Dutch translation
A number of problems were identified with the Dutch translation of the NDI. First, the patient instruction that precedes the original version of the NDI is not included in the NDI-DV. Second, for some items, the Dutch translation (or part of it) was considered inadequate. For example, the Dutch translation of the phrase “… but it causes extra neck pain” in the item “personal care” was back translated into English as “although this increases the pain.” Saying that personal care causes extra neck
Discussion
The most commonly used Dutch translation of the NDI was considered to be suboptimal. Poor translation implies that scores on the Dutch and the English versions of the NDI might not be comparable. It may also affect content validity if translated items are interpreted differently.
Many researchers interpret the questionnaire as a measure of physical functioning [7], [8]. However, our study shows that the NDI measures more than physical functioning. Review of the literature and personal
Conclusion
It is unclear what the NDI aims to measure. It measures more than physical functioning. However, for a broader construct, important items are missing. The content validity of the NDI is poor. An important lesson from this study is that a clear definition of the construct to be measured is a prerequisite for the assessment of validity. In line with current recommendations, for example, from the US Food and Drug Administration [33], we conclude that more attention should be given to the
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The authors declare that they have no competing interest. Luc Ailliet, the corresponding author, is as a PhD student financially supported by The European Chiropractic Union, the Belgian Chiropractic Union, The Netherlands Chiropractic Association (NCA), and l’Institut Franco-Européen de Chiropratique (IFEC, Paris, France). The views expressed in this manuscript are those of the authors, and no official endorsements by supporting agencies is intended or should be inferred.