The authors respond

Abstract

We appreciate the comments of Vernon on our analysis of the psychometric properties of the Neck Disability Index (NDI) and Numeric Pain Rating Scale in patients with neck pain, 1 Cleland J.A. Childs J.D. Whitman J.M. Psychometric properties of the Neck Disability Index and Numeric Pain Rating Scale in patients with mechanical neck pain. Arch Phys Med Rehabil. 2008; 89: 69-74 Abstract Full Text Full Text PDF PubMed Scopus (423) Google Scholar and we would graciously take this opportunity to respond to his concerns. Vernon states that previous studies have reported that the test-retest reliability is considerably higher than that we identified in our analysis (.50). However, one of his cited references 2 Cook C. Richardson J.K. Braga L. et al. Cross-cultural adaptation and validation of the Brazilian Portuguese version of the Neck Disability Index and Neck Pain and Disability Scale. Spine. 2006; 31: 1621-1627 Crossref PubMed Scopus (164) Google Scholar investigated the test-retest reliability of the Brazilian Portuguese version of the NDI and identified a reliability coefficient of .48 when the instrument was retested over a 7-day period. Vernon also questions our use of a before-and-after treatment “stable” group as a proxy representation of test-retest reliability. We disagree that the 2 testing sessions must be “similar in all … respects with the exception of some interval of passage of time. …” Rather, we argue that the most important aspect when assessing test-retest reliability is assuring that the response variable remains stable. 3 Portney L.G. Watkins M.P. Foundations of clinical research: applications to practice. 2nd ed. Prentice Hall Health, Upper Saddle River2000 Google Scholar We cannot be confident that a patient's status will not change over time, even in the absence of an intervention. Vernon also references the Vos study, 4 Vos C.J. Verhagen A.P. Koes B.W. Reliability and responsiveness of the Dutch version of the Neck Disability Index in patients with acute neck pain in general practice. Eur Spine J. 2006; 15: 1729-1736 Crossref PubMed Scopus (86) Google Scholar which used a perceived recovery scale as a construct to determine stability in the group for test-retest purposes similar to our study. We recognize that a variety of test-retest methods have been used and that further research is necessary to establish optimal research methods; however, our approach using an external criterion for defining a “stable” group has been widely used in previous research assessing the test-retest reliability of self-report measures of pain and disability. 5 Fritz J.M. Irrgang J.J. A comparison of a modified Oswestry Low Back Disability Questionnaire and the Quebec Back Pain Disability Scale. Phys Ther. 2001; 81: 776-788 Crossref PubMed Scopus (706) Google Scholar , 6 Binkley J.M. Stratford P.W. Lott S.A. Riddle D.L. North American Orthopaedic Rehabilitation Research NetworkThe Lower Extremity Functional Scale (LEFS): scale development, measurement properties, and clinical application. Phys Ther. 1999; 79: 371-383 PubMed Google Scholar , 7 Bendebba M. Heller J. Ducker T.B. Eisinger J.M. Cervical spine outcomes questionnaire: its development and psychometric properties. Spine. 2002; 27: 2116-2123 Crossref PubMed Scopus (86) Google Scholar , 8 Westaway M. Stratford P. Binkley J. The Patient-Specific Functional Scale: validation of its use in persons with neck dysfunction. J Orthop Sports Phys Ther. 1998; 27: 331-338 PubMed Google Scholar , 9 Chansirinukor W. Maher C.G. Latimer J. Hush J. Comparison of the functional rating index and the 18-item Roland-Morris Disability Questionnaire: responsiveness and reliability. Spine. 2005; 30: 141-145 PubMed Google Scholar The Psychometric Properties of the Neck Disability IndexArchives of Physical Medicine and RehabilitationVol. 89Issue 7PreviewThe findings of Cleland et al1 with respect to the psychometric properties of the Neck Disability Index (NDI) differ greatly with previously published work. These anomalous findings require explication beyond what is offered by Cleland. Full-Text PDF