Agreement between original and Rasch-approved neck disability index

Table 2 Bland-Altman statistics and non-parametric comparisons by Wilcoxon signed rank test

Comparison	Sample size	Degree of freedom	Individual agreement				Average agreement	Wilcoxon signed rank test
Comparison	Sample size	Degree of freedom	Mean of difference (d) with (95% CI)	SD of difference	Upper LoA d + 1.96SD	Lower LoA d-1.96SD	SE	Wilcoxon signed rank test
NDI-10 vs. NDI-8	201	200	−2.3* (−3.0 - -1.6)	5.0	7.4	−12.0	0.4	P < 0.001*
NDI-10 vs. NDI-5	201	200	−4.6* (−5.3 - -3.8)	5.3	5.8	−14.9	0.5	P < 0.001*
NDI-8 vs. NDI-5	201	200	−2.3* (−2.7 - -1.9)	2.8	3.3	−7.8	0.2	P < 0.001*

* significant as p < 0.001
NDI-10: The total score of NDI 10-item (original) version on ordinal scale with maximum of 50 points
NDI-8: The total score of NDI 8-item version on linear scale with maximum of 50 points
NDI-5: The total score of NDI 5-item version on linear scale with maximum of 50 points
SD standard deviation
SE standard error base on the mean of difference

ISSN: 1471-2288