The reliability of the Vernon and Mior neck disability index, and its validity compared with the short form-36 health survey questionnaire

Prospective single cohort study. To evaluate the NDI by comparison with the SF36 health Survey Questionnaire. The NDI is a simple ten-item questionnaire used to assess patients with neck pain. The SF36 measures functional ability, well being and the overall health of patients. It is used as a gold standard in health economics to assess the health utility, gain and economic impact of medical interventions. One hundred and sixty patients with neck pain attending the spinal clinic completed self-assessment questionnaires. A second questionnaire was completed in 34 patients after a period of 1-2 weeks. The internal consistency of the NDI and SF36 was calculated using Cronbach's alpha. The test-retest reliability was assessed using the Bland and Altman method. The concurrent validity of the NDI with respect to the SF-36 was assessed using Pearson correlations. Both questionnaires showed robust internal consistency: Cronbach's alpha for the NDI scale was acceptable (0.864, 95% confidence limits 0.825-0.894) though slightly smaller than that of the SF36. The correlations between each item of the NDI scores and the total NDI score ranged from 0.447 to 0.659, (all with P < 0.001). The test-retest reliability of the NDI was high (intra-class correlation 0.93, 95% confidence limits 0.86-0.97) and comparable with the best values found for SF36. The correlations between NDI and SF36 domains ranged from -0.45 to -0.74 (all with P < 0.001). We have shown that the NDI has good reliability and validity and that it compares well with the SF36 in the spinal surgery out patient setting.