Aberdeen Health Psychology Group, University of Aberdeen, Aberdeen, UK.
BMC Musculoskelet Disord. 2012 Dec 29;13:265. doi: 10.1186/1471-2474-13-265.
The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) is a widely used patient reported outcome in osteoarthritis. An important, but frequently overlooked, aspect of validating health outcome measures is to establish if items exhibit differential item functioning (DIF). That is, if respondents have the same underlying level of an attribute, does the item give the same score in different subgroups or is it biased towards one subgroup or another. The aim of the study was to explore DIF in the Likert format WOMAC for the first time in a UK osteoarthritis population with respect to demographic, social, clinical and psychological factors.
The sample comprised a community sample of 763 people with osteoarthritis who participated in the Somerset and Avon Survey of Health. The WOMAC was explored for DIF by gender, age, social deprivation, social class, employment status, distress, body mass index and clinical factors. Ordinal regression models were used to identify DIF items.
After adjusting for age, two items were identified for the physical functioning subscale as having DIF with age identified as the DIF factor for 2 items, gender for 1 item and body mass index for 1 item. For the WOMAC pain subscale, for people with hip osteoarthritis one item was identified with age-related DIF. The impact of the DIF items rarely had a significant effect on the conclusions of group comparisons.
Overall, the WOMAC performed well with only a small number of DIF items identified. However, as DIF items were identified in for the WOMAC physical functioning subscale it would be advisable to analyse data taking into account the possible impact of the DIF items when weight, gender or especially age effects, are the focus of interest in UK-based osteoarthritis studies. Similarly for the WOMAC pain subscale in people with hip osteoarthritis it would be worthwhile to analyse data taking into account the possible impact of the DIF item when age comparisons are of primary interest.
安大略西部和麦克马斯特大学骨关节炎指数（WOMAC）是一种广泛应用于骨关节炎的患者报告结局。验证健康结果测量的一个重要但经常被忽视的方面是确定项目是否存在差异功能（DIF）。也就是说，如果受访者具有相同的潜在属性水平，那么项目在不同亚组中的得分是否相同，或者它是否偏向于一个亚组或另一个亚组。本研究的目的是首次在英国骨关节炎人群中探讨 Likert 格式 WOMAC 的 DIF，涉及人口统计学、社会、临床和心理因素。
该样本包括 763 名患有骨关节炎的社区参与者，他们参加了萨默塞特和埃文健康调查。通过性别、年龄、社会贫困、社会阶层、就业状况、痛苦、体重指数和临床因素探讨 WOMAC 的 DIF。使用有序回归模型识别 DIF 项目。
调整年龄后，两个项目被确定为身体功能子量表存在 DIF，年龄被确定为 2 个项目的 DIF 因素，性别为 1 个项目，体重指数为 1 个项目。对于 WOMAC 疼痛子量表，对于患有髋关节炎的人，有 1 个项目被确定为与年龄相关的 DIF。DIF 项目的影响很少对组间比较的结论产生重大影响。
总体而言，WOMAC 表现良好，仅发现少量 DIF 项目。然而，由于在 WOMAC 身体功能子量表中发现了 DIF 项目，当体重、性别或特别是年龄影响成为英国骨关节炎研究的关注焦点时，分析数据时考虑 DIF 项目的可能影响是明智的。同样，对于患有髋关节炎的人，在 WOMAC 疼痛子量表中，当年龄比较是主要关注点时，考虑 DIF 项目的可能影响是值得的。
