Differential item functioning in quality of life measure between children with and without special health-care needs.

Objectives: Limited studies consider the effect of differential item functioning (DIF) on health-related quality of life (HRQOL) comparisons between ill and health children. The objective is to assess DIF and compare HRQOL between children with special health-care needs (CSHCN) and children without needs.

Methods: Data were collected from 1195 families of children enrolled in Florida's public insurance programs. HRQOL was measured using physical, emotional, social, and school functioning of the Pediatric Quality of Life Inventory (PedsQL). We identified CSHCN using the CSHCN Screener and assessed DIF related to CSHCN using a multiple group-multiple indicators-multiple causes method (MG-MIMIC). We assessed the impact of DIF by examining expected item/test scores and item/test information function. We tested the discrepancy between underlying HRQOL scores of both groups before and after DIF calibration (allowing parameters of DIF items to be different and DIF-free items to be the same across both groups).

Results: Two (25%) and three items (60%) of physical and school functioning, respectively, were identified with nonuniform DIF, and two items (40%) of social functioning were identified with uniform DIF. Expected item/test scores and item/test information function suggest that the impact of DIF is minimal. Before DIF calibration, HRQOL in CSHCN was more impaired than in children without needs (effect sizes -1.04, -0.74, -0.96, and -0.98 for physical, emotional, social, and school functioning, respectively). After DIF calibration, the discrepancy was increased slightly.

Conclusions: Although 30% of items on the PedsQL were identified with DIF related to CSHCN status, the impact of DIF is minimal.