Rasch analysis provides new insights into the measurement properties of the neck disability index
Article first published online: 30 MAR 2009
Copyright © 2009 by the American College of Rheumatology
Arthritis Care & Research
Volume 61, Issue 4, pages 544–551, 15 April 2009
How to Cite
van der Velde, G., Beaton, D., Hogg-Johnston, S., Hurwitz, E. and Tennant, A. (2009), Rasch analysis provides new insights into the measurement properties of the neck disability index. Arthritis & Rheumatism, 61: 544–551. doi: 10.1002/art.24399
- Issue published online: 30 MAR 2009
- Article first published online: 30 MAR 2009
- Manuscript Accepted: 14 JAN 2009
- Manuscript Received: 3 MAY 2008
- Fellowship award
- Bisby award from the Canadian Institutes of Health Research
- New Investigator award from the Canadian Institutes of Health Research
The most widely used neck-specific measure in intervention trials is the 10-item Neck Disability Index (NDI), which is assumed to be a unidimensional interval scale, as shown by how NDI data are scored, analyzed, and interpreted. Our objective was to use modern measurement methods to test this assumption (and thereby to also test the validity of calculating summed scores and parametric statistics on NDI data) through Rasch analysis.
NDI data from 521 trial subjects with neck pain were fit to the Rasch model. We examined threshold ordering of NDI items, fit of data to model expectations, presence of differential item functioning, and whether or not the set of NDI items collectively measure a single construct, which is a requirement for calculating summative scores.
There was a lack of fit of data to the Rasch model (χ2 = 140.35, 70 df; P < 0.001). Five items (personal care, lifting, headaches, work, and recreation) had disordered response thresholds. Differential item functioning was detected for age and sex. The NDI items did not contribute to a single construct. Unidimensionality and interval scaling were achieved by removing 2 of the 10 items (resulting in the NDI-8), and converting NDI-8 ordinal (paper) summative scores to NDI-8 interval (Rasch-weighted) scores.
As originally proposed and conventionally used, the NDI is not a unidimensional scale, and has only ordinal scaling. This raises fundamental doubts about the practice of calculating change scores and other parametric statistics on NDI data. A revised 8-item version provides unidimensional interval-level measurement of neck pain disability.