Department of Health Economics, Corvinus University of Budapest, 8 Fővám tér, 1093, Budapest, Hungary.
Hungarian Academy of Sciences, Premium Postdoctoral Research Programme, 7 Nádor u, 1051, Budapest, Hungary.
Qual Life Res. 2021 Aug;30(8):2375-2386. doi: 10.1007/s11136-021-02803-7. Epub 2021 Mar 8.
Eight of the ten items of the Dermatology Life Quality Index (DLQI) have a 'not relevant' response (NRR) option. There are two possible ways to interpret NRRs: they may be considered 'not at all' or missing responses. We aim to compare the measurement performance of the DLQI in psoriasis patients when NRRs are scored as '0' (hereafter zero-scoring) and 'missing' (hereafter missing-scoring) using Rasch model analysis.
Data of 425 patients with psoriasis from two earlier cross-sectional surveys were re-analysed. All patients completed the paper-based Hungarian version of the DLQI. A partial credit model was applied. The following model assumptions and measurement properties were tested: dimensionality, item fit, person reliability, order of response options and differential item functioning (DIF).
Principal component analysis of the residuals of the Rasch model confirmed the unidimensional structure of the DLQI. Person separation reliability indices were similar with zero-scoring (0.910) and missing-scoring (0.914) NRRs. With zero-scoring, items 6 (sport), 7 (working/studying) and 9 (sexual difficulties) suffered from item misfit and item-level disordering. With missing-scoring, no misfit was observed and only item 7 was illogically ordered. Six and three items showed DIF for gender and age, respectively, that were reduced to four and three by missing-scoring.
Missing-scoring NRRs resulted in an improved measurement performance of the scale. DLQI scores of patients with at least one vs. no NRRs cannot be directly compared. Our findings provide further empirical support to the DLQI-R scoring modification that treats NRRs as missing and replaces them with the average score of the relevant items.
皮肤病生活质量指数（DLQI）的十个项目中有八个具有“不相关”（NRR）选项。有两种可能的方法来解释 NRR：它们可以被视为“完全不”或缺失的回答。我们旨在通过 Rasch 模型分析比较 NRR 被评分“0”（以下简称零分）和“缺失”（以下简称缺分）时，在银屑病患者中使用 DLQI 的测量性能。
重新分析了来自两项早期横断面调查的 425 例银屑病患者的数据。所有患者均完成了基于纸张的匈牙利版 DLQI。应用部分信用模型。测试了以下模型假设和测量特性：维度、项目拟合、人员可靠性、反应选项顺序和差异项目功能（DIF）。
Rasch 模型残差的主成分分析证实了 DLQI 的单维结构。零分（0.910）和缺分（0.914）NRR 的人员分离可靠性指数相似。在零分情况下，第 6 项（运动）、第 7 项（工作/学习）和第 9 项（性困难）的项目拟合和项目水平紊乱。在缺分情况下，没有观察到不拟合，只有第 7 项的逻辑顺序不合理。第 6 项和第 3 项显示出性别和年龄的 DIF，分别通过缺分减少到 4 项和 3 项。
缺分 NRR 导致了量表测量性能的提高。至少有一个 NRR 与没有 NRR 的患者的 DLQI 评分不能直接比较。我们的发现为将 NRR 视为缺失并以相关项目的平均分数替换它们的 DLQI-R 评分修改提供了进一步的经验支持。
