Differential item functioning by language on the PROMIS® physical functioning items for children and adolescents.

PURPOSE: To assess the equivalence of self-reports of physical functioning between pediatric respondents to the English- and Spanish-language patient-reported outcomes measurement information system (PROMIS®) physical functioning item banks.
METHODS: The PROMIS pediatric physical functioning item banks include 29 upper extremity items and 23 mobility items. A sample of 5091 children and adolescents (mean age = 12 years, range 8-17; 49% male) completed the English-language version of the items. A sample of 605 children and adolescents (mean age = 12 years, range 8-17; 55% male; 96% Hispanic) completed the Spanish-language version of the items.
RESULTS: We found language (English versus Spanish) differential item functioning (DIF) for 4 upper extremity items and 7 mobility items. Product-moment correlations between estimated upper extremity and mobility scores using the English versus the equated Spanish item parameters for Spanish-language respondents were 0.98 and 0.99, respectively. After excluding cases with significant person misfit, we found DIF for the same 4 upper extremity items that had DIF in the full sample and for 12 mobility items (including the same 7 mobility items that had DIF in the full sample). The identification of DIF items between English- and Spanish-language respondents was affected slightly by excluding respondents displaying person misfit.
CONCLUSIONS: The results of this study provide support for measurement equivalence of self-reports of physical functioning by children and adolescents who completed the English- and Spanish-language surveys. Future analyses are needed to replicate the results of this study in other samples.