Agreement between Original and Rasch-Approved Neck Disability Index

Abstract Background: The neck disability index (NDI) has been used widely to monitor the treatment outcomes from patients’ perspective, given that neck pain is a notable social burden with a high prevalence (33%) within the adult population. Previous researchers have examined its psychometric properties and latent construct. Uptake of modified versions of NDI have been limited, however, the traditional/original NDI is still commonly administered. Examining the extent of agreement between traditional and Rasch-based versions using Bland-Altman (B&A) plots will inform our understanding of score differences. Therefore, the objective of current study was to describe the extent of agreement between different versions of NDI.Methods: The current study was a secondary data analysis where the study data was compiled from two prospectively collected data source. We performed a comprehensive literature search to identify Rasch analyses of the NDI within four databases including Embase, Medline, PubMed, and Google Scholar. Modified version with associate score transformation solution was obtained if they meet the predefined criteria. We established B&A plot by the mean difference and the 95% limits of agreement (LoA; ±1.96 times the standard deviation).Results: Two Rasch approved versions (8- and 5- item) that met the study criteria were identified from 303 publications. We analyzed 201 (43 males and 158 females) patients attending community clinics for neck pain. We found that the mean difference was approximately 10% of the total score between the 10-item and 5-item (-4.6 points), whereas the 10-item versus 8-item and 8-item versus 5-item had similar mean differences that were about half (-2.3 points). The B&A plots displayed wider 95% LoA for the agreement between 10-item and 8-item (LoA: -12.0, 7.4) and 5-item (LoA: -14.9, 5.8) compared with the agreement of 8-item and 5-item (LoA: -7.8, 3.3). Conclusion: Our study is the first to examine the agreement between each versions of NDI. Due to the unpredictable bias and wide 95% LoA, the traditional 10-item version should not be used interchangeably with other two Rasch-approved versions even with a small magnitude of bias. Different coverage of content should be considered during the decision of 8- and 5-item NDI.