Rasch analysis of the Persian version of PedsQL(TM) Oral Health Scale: further psychometric evaluation on item validity including differential item functioning

Background: The study aimed to further evaluate the psychometric properties of one recently developed oral health related quality of life (OHRQoL) instrument (PedsQL Oral Health Scale), including student self-report and parent-proxy report. Specifically, we tested the item validity,threshold order, local dependency, and differential item functioning (DIF) across gender and rater. Methods: This is a cross-sectional study, and study population was recruited in Qazvin, Iran using one-stage sampling with the unit of school. Students and their parents (1529 dyads) separately completed the Persian version of PedsQL Oral Health Scale. The psychometric properties were analyzed using Rasch rating scale model, including item validity, threshold order for response categories, and DIF across gender (boys vs. girls in student self-report) and rater (student self report vs. parent-proxy report). Results: All items had satisfactory in fit and outfit mean square error. One disordering category (the response of often) was found in parent-proxy report, while all categories were ordered in student self-report. All items were DIF-trivial across gender and rater. Conclusion: PedsQL Oral Health Scale is a valid instrument to measure OHRQoL. However, our results indicated that the parent-proxy report was inferior to the student self-report, and healthcare providers should primarily use the student self-report.