Rasch analysis provides new insights into the measurement properties of the neck disability index

Authors

  • Gabrielle van der Velde,

    Corresponding author
    1. Toronto Health Economics and Technology Assessment Collaborative, University of Toronto, Toronto General Research Institute, University Health Network, Institute for Work & Health, Centre of Research Expertise in Improved Disability Outcomes, University Health Network Rehabilitation Solutions, Toronto Western Hospital, Toronto, Ontario, Canada
    • Toronto Health Economics and Technology Assessment Collaborative, Leslie Dan Pharmacy Building, University of Toronto, 6th Floor, Room 658, 144 College Street, Toronto, Ontario, Canada M5S 3M2
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  • Dorcas Beaton,

    1. Institute for Work & Health, University of Toronto, and Li Ka-Shing Knowledge Institute of St. Michael's Hospital, Toronto, Ontario, Canada
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  • Sheilah Hogg-Johnston,

    1. Institute for Work & Health, and the University of Toronto, Toronto, Ontario, Canada
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  • Eric Hurwitz,

    1. University of Hawaii at Mānoa, Honolulu
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  • Alan Tennant

    1. University of Leeds, Leeds, UK
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Abstract

Objective

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.

Methods

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.

Results

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.

Conclusion

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.

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