Quantifying 'Problematic' DIF Within an IRT Framework

Application to a Cancer Stigma Index

Published In: Quality of Life Research, v. 24, no. 1, Jan. 2015, p. 95-103

Posted on RAND.org on January 01, 2013

by Maria Orlando Edelen, Brian D. Stucky, Anita Chandra

Read More

Access further information on this document at Springer

This article was published outside of RAND. The full text of the article can be found at the link above.

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.

This report is part of the RAND Corporation External publication series. Many RAND studies are published in peer-reviewed scholarly journals, as chapters in commercial books, or as documents published by other organizations.

Our mission to help improve policy and decisionmaking through research and analysis is enabled through our core values of quality and objectivity and our unwavering commitment to the highest level of integrity and ethical behavior. To help ensure our research and analysis are rigorous, objective, and nonpartisan, we subject our research publications to a robust and exacting quality-assurance process; avoid both the appearance and reality of financial and other conflicts of interest through staff training, project screening, and a policy of mandatory disclosure; and pursue transparency in our research engagements through our commitment to the open publication of our research findings and recommendations, disclosure of the source of funding of published research, and policies to ensure intellectual independence. For more information, visit www.rand.org/about/principles.

The RAND Corporation is a nonprofit institution that helps improve policy and decisionmaking through research and analysis. RAND's publications do not necessarily reflect the opinions of its research clients and sponsors.