Psychometric properties of the neck disability index amongst patients with chronic neck pain using item response theory

Abstract

Objective: The Neck Disability Index (NDI) is commonly used for clinical and research assessment for chronic neck pain, yet the original version of this tool has not undergone significant validity testing, and in particular, there has been minimal assessment using Item Response Theory. The goal of the present study was to investigate the psychometric properties of the original version of the NDI in a large sample of individuals with chronic neck pain by defining its internal consistency, construct structure and validity, and its ability to discriminate between different degrees of functional limitation.

Methods: This is a cross-sectional cohort study of 585 consecutive patients with chronic neck pain seen in a university hospital rehabilitation clinic. Internal consistency was evaluated using Cronbach’s alpha, construct structure was evaluated by exploratory factor analysis, and discrimination ability was determined by Item Response Theory.

Results: The NDI demonstrated good internal consistency assessed by Cronbach’s alpha (0.87). The exploratory factor analysis identified only one factor with eigenvalue considered significant (cutoff 1.0). When analyzed by Item Response Theory, eight out of 10 items demonstrated almost ideal difficulty parameter estimates. In addition, eight out of 10 items showed high to perfect estimates of discrimination ability (overall range 0.8 to 2.9).

Conclusions: Amongst patients with chronic neck pain, the NDI was found to have good internal consistency, have unidimensional properties, and an excellent ability to distinguish patients with different levels of perceived disability.

Implications for Rehabilitation

• The Neck Disability Index has good internal consistency, unidimensional properties, and an excellent ability to distinguish patients with different levels of perceived disability.

• The Neck Disability Index is recommended for use when selecting patients for rehabilitation, setting rehabilitation goals, and measuring the outcome of intervention.

Keywords:

• Neck pain
• neck disability index
• NDI
• Rasch
• item response theory
• validity

Disclosure statement

Zachary McCormick, MD serves on the Board of Directors of the Spine Intervention Society. There are no other conflicts of interest to declare.