Assessing whether parents and children perceive the meaning of the items in the PedsQLTM 4.0 quality of life instrument consistently: a differential item functioning analysis.

Limited studies have examined the effect of differential item functioning (DIF) on comparing health related quality of life (HRQoL) scores across child self-reports and parent proxy-reports. This study aims to determine whether parents and children respond differently to the items in the Persian version of the PedsQoLTM 4.0 measure. The PedsQLTM 4.0 Generic Core Scales was completed by 938 child-parent dyads. The graded response model (GRM) was used to detect DIF between parents and children. The IRT analyses were conducted using IRTPRO 2.1.On the whole, our findings showed that 50% (4 out of 8) of the items in the physical subscale and 40% (2 out of 5) in both emotional and school subscales were flagged with DIF. Among the DIF items, 62.5% (5 out of 8) were uniform and the remaining 37.5% (3 out of 8) were non-uniform. Parents and children interpret certain items of the PedsQLTM 4.0 in a different ways, except for the social subscale. Hence, we should be cautious about using parent proxy-report as a substitute for a child's ratings.