Abstract
Differential item functioning (DIF) based on Rasch model can be used to examine whether the items function similarly across different groups and identify items that appear to be too easy or difficult after controlling for the ability levels of the compared groups. The Oswestry low back pain disability (Oswestry) has traditionally been proved as an effective instrument measuring disability resulting from low back pain (LBP). In this study, DIF method was used to explore whether items on the Oswestry perform similarly across two different groups (participants with LBP and no LBP). A series of Rasch analyses on the 10 items of the Oswestry were performed using Winsteps$^{(R)}$ software. Forty-two participants with back pain were recruited from 3 rehabilitation hospitals in Gainesville, Florida. Another 42 participants with no LBP were recruited from several public places in the rehabilitation hospitals. Based on the DIF analysis across the two groups, several items were found to have an uniform DIF. Participants with no LBP had more difficulty on lifting and personal care items and participants with LBP had more difficulty on sleeping and social life items. For non-LBP group, a high ceiling effects (83% of participants with non-LBP) was detected, which was not be able to be effectively measured with the Oswestry items. Although 4 items of the Oswestry function differently across the two groups, all items of the Oswestry were well targeted the LBP group.
