Differential item functioning (DIF)
Finally, when comparing subjects by gender, four items (Item 1, Item 8, Item 9 and Item 15) showed differential item functioning. Items 1 (“My choice of clothing is affected”), 8 (“I feel embarrassed”), and 9 (“I feel frustrated”) with a DIF contrast of 0.70, 0.77, and 0.60, respectively, had a moderate to large impact (category C). Item 15 (“I avoid meeting new people”) with a DIF contrast value of − 0.48 was in category B, indicating a mild to moderate impact. It referred to avoiding new people due to the condition and was more symptomatic for men. Before testing for differential item functioning for age, we divided the sample into two subgroups. The discriminatory criterion was the medium age of 33 years in order to divide the sample in two groups, one representing younger patients and one composed of adults and older patients. Both groups accounted for approximately 50% of the original sample and were thus suitable for a valid comparison of the response behavior of these subgroups. When testing for differential item functioning for age three items were significant. Item 7 (“I feel nervous”) and Item 12 (“I worry about my future health”) were both in category B with a DIF contrast of − 0.54 and 0.53, respectively. Item 14 (“I worry about leaving sweat marks on things”) had a DIF contrast of − 1.12 (category C).