Purpose: This study further investigated items with differential item function (DIF) in the Social/Cognitive domain of a measure of everyday activity performance, the Pediatric Evaluation of Disability Inventory-Computer Adapted Test version for Autism "PEDI-CAT (ASD)," to understand possible sources of response variation in a heterogeneous sample of youth with autism compared to the national standardization sample.
Methods: Cross-sectional design. A convenience sample of parents who identified they had a child between 3 and 21 years (M = 11.9 years, SD = 4.67 years) with autism (n = 365) completed an online survey that included the PEDI-CAT (ASD) and descriptive measures. For 28 items previously identified as having DIF, the PEDI-CAT (ASD) expected item score curves for the autism sample were compared to the original PEDI-CAT standardization sample. The weighted area between expected score curves (wABC) was also calculated; values >0.24 indicate significant DIF.
Results: All items had wABC that exceeded the criterion. Compared with peers without disabilities at the same ability level, 11 items were significantly more difficult for the youth with autism and 16 items were significantly easier. One item demonstrated non-uniform DIF.
Conclusion: Differential responses could indicate that: (1) children with autism have a different developmental pattern of skill acquisition for everyday activities in the Social/Cognitive domain, or (2) parents of children with autism utilize a unique appraisal process when assessing their children's functional performance of everyday activities. Further research is required to better understand the factors leading to differential responses on the targeted items. The study illustrates the value of in-depth analysis of DIF to gain insight into the impact of a clinical condition on functional performance.
Keywords: Autistic disorder; Differential item function (DIF); Item response theory.