BACKGROUND An overarching objective in research comparing different sample groups is to ensure that the reported differences in outcomes are not affected by differences between groups in the functioning of the measurement instruments, i.e. the items have to work in the same way for the different sample groups to be compared. Lack of invariance across sample groups are commonly called Differential Item Functioning (DIF). There is a sense in which the DIF of an item can be taken account of by resolving (splitting) the item into group specific items, rather than deleting the item. Resolving improves fit, retains the reliability and content provided by the item, and compensates for the DIF in estimation of person parameters on the scale of the instrument. However, it destroys invariance of the item's parameter value among the groups. Whether or not a DIF item should be resolved depends on whether the source of the DIF is relevant or irrelevant for the content of the variable. The present paper shows how external information can be used to investigate if the gender DIF found in the item "Stomach ache" in a psychosomatic symptoms scale used among adolescents may reflect abdominal pain because of a biological factor, the girls' menstrual periods. METHODS Swedish data from the international Health Behaviour in School-aged Children study (HBSC) collected in 2005/06, 2009/10 and 2013/14 were used, comprising a total of 18,983 students in grades 5, 7 and 9. A composite measure of eight items of psychosomatic problems was analysed for DIF with respect to gender and menstrual periods using the Rasch model. RESULTS The results support the hypothesis that the source of the gender DIF for the item "Stomach ache" is a gender specific biological factor. In that case the DIF should be resolved if the psychosomatic measure is not intended to tap information about abdominal pain caused by a gender specific biological factor. In contrast, if the measure is intended to tap such information, the DIF should not be resolved. CONCLUSIONS The conceptualisation of the measure governs whether the item showing DIF should be resolved or not.

中文翻译：

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解释差异项目的功能，着重于外部信息的关键作用-来自青少年心理健康衡量的一个例子。

背景技术比较不同样本组的研究的首要目标是确保报告的结果差异不受测量工具功能方面各组之间的差异的影响，即不同样本组的项目必须以相同的方式工作。进行比较。样本组之间缺乏不变性通常称为差异项功能（DIF）。从某种意义上说，可以通过将项目解析（拆分）为特定于组的项目而不是删除该项目来考虑该项目的DIF。分辨率提高了合身性，保留了该项目提供的可靠性和内容，并补偿了在仪器规模上的人为参数估计中的DIF。但是，它破坏了组之间项目参数值的不变性。DIF项目是否应解析取决于DIF的来源与变量的内容相关还是无关。本文显示了如何使用外部信息调查在青少年中使用的心身症状量表中的“胃痛”项中发现的性别DIF是否可能反映出由于生物学因素（即女孩的月经期）而引起的腹痛。方法使用瑞典于2005 / 06、2009 / 10和2013/14年度国际学龄儿童健康行为研究（HBSC）收集的数据，共计18,983名5、7和9年级学生。使用Rasch模型分析了八种心身问题的性别和月经期间的DIF。结果该结果支持以下假设：“胃痛”项目的性别DIF来源是性别特定的生物学因素。在这种情况下，如果心身测验的目的不是要利用有关性别特定生物学因素引起的腹痛的信息，则应解决DIF。相反，如果该措施旨在利用此类信息，则不应解决DIF。结论措施的概念化决定了是否应该解决显示DIF的项目。如果该措施旨在利用此类信息，则不应解决DIF。结论措施的概念化决定了是否应该解决显示DIF的项目。如果该措施旨在利用此类信息，则不应解决DIF。结论措施的概念化决定了是否应该解决显示DIF的项目。