Quantifying 'Problematic' DIF Within an IRT Framework

PURPOSE: Differential item functioning (DIF) detection within an item response theory (IRT) framework is highly powerful, often identifying significant DIF that is of little clinical importance. This paper introduces two metrics for IRT DIF evaluation that can discern potentially problematic DIF among items flagged with statistically significant DIF. METHODS: Computation of two DIF metrics — (1) a weighted area between the expected score curves (wABC) and (2) a difference in expected a posteriori scores across item response categories (dEAP) — is described. Their use is demonstrated using data from a 27-item cancer stigma index fielded to four adult samples: (1) Arabic (N = 633) and (2) English speakers (N = 324) residing in Jordan and Egypt, and (3) English (N = 500) and (4) Mandarin speakers (N = 500) residing in China. We used IRTPRO's DIF module to calculate IRT-based Wald chi-square DIF statistics according to language within each region. After standard p value adjustments for multiple comparisons, we further evaluated DIF impact with wABC and dEAP. RESULTS: There were a total of twenty statistically significant DIF comparisons after p value adjustment. The wABCs for these items ranged from 0.13 to 0.90. Upon inspection of curves, DIF comparisons with wABCs >0.3 were deemed potentially problematic and were considered further for removal. The dEAP metric was also informative regarding impact of DIF on expected scores, but less consistently useful for narrowing down potentially problematic items. CONCLUSIONS: The calculations of wABC and dEAP function as DIF effect size indicators. Use of these metrics can substantially augment IRT DIF evaluation by discerning truly problematic DIF items among those with statistically significant DIF.