Differential Item Functioning of the Jaw Functional Limitation Scale

Aims: To assess the differential item functioning (DIF) of the Jaw Functional Limitation Scale (JFLS) due to gender, age, and language (English vs Spanish).
Methods: JFLS data were collected from a consecutive sample of 2,115 adult dental patients from HealthPartners dental clinics in Minnesota. Participants with missing data were excluded, and analyses were performed using data from 1,678 participants. Whether the item response theory (IRT) model assumptions of essential unidimensionality and local independence held up for the JFLS was examined. Then, using Samejima's graded response model, the IRT log-likelihood ratio approach was used to detect DIF. The magnitude and impact of DIF based on Raju's noncompensatory DIF (NCDIF) cutoff value of 0.096, Cohen's effect sizes, and test (or scale) characteristic curves were also assessed.
Results: Essential unidimensionality was confirmed, but locally dependent items were found on the JFLS. A few items were flagged with statistically significant DIF after adjustment for multiple comparisons. The NCDIF indices associated with all DIF items were < 0.096, and they had small effect sizes of ≤ 0.2. The differences between the expected scores shown in the test characteristic curves were little to none.
Conclusion: The present results support the use of the JFLS summary score to obtain psychometrically robust score comparisons across English- and Spanish-speaking, male and female, and younger and older dental patients. Overall, the magnitude of DIF was relatively small, and the practical impact minimal.