van der Velde Gabrielle, Beaton Dorcas, Hogg-Johnston Sheilah, Hurwitz Eric, Tennant Alan
Toronto Health Economics and Technology Assessment Collaborative, University of Toronto, Toronto, Canada.
Arthritis Rheum. 2009 Apr 15;61(4):544-51. doi: 10.1002/art.24399.
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.
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.
There was a lack of fit of data to the Rasch model (chi(2) = 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.
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.
干预试验中最广泛使用的颈部特异性测量工具是10项颈部功能障碍指数（NDI），从NDI数据的评分、分析和解释方式来看，它被假定为单维等距量表。我们的目的是使用现代测量方法，通过Rasch分析来检验这一假设（从而也检验对NDI数据计算总分和参数统计的有效性）。
将来自521名颈部疼痛试验受试者的NDI数据拟合到Rasch模型。我们检查了NDI项目的阈值排序、数据与模型预期的拟合度、项目功能差异的存在情况，以及NDI项目集是否共同测量单一结构，这是计算总分的一个要求。
数据与Rasch模型拟合不佳（卡方=140.35，自由度70；P<0.001）。五个项目（个人护理、提举、头痛、工作和娱乐）的反应阈值无序。检测到年龄和性别的项目功能差异。NDI项目并非对单一结构有贡献。通过去除10个项目中的2个（得到NDI-8），并将NDI-8的序数（纸质）总分转换为NDI-8的等距（Rasch加权）分数，实现了单维性和等距标度。
按照最初提出和传统使用的方式，NDI不是单维量表，只有序数标度。这对计算NDI数据的变化分数和其他参数统计的做法提出了根本性的质疑。修订后的8项版本提供了颈部疼痛残疾的单维等距水平测量。
