Detecting Dif for Polytomously Scored Items: An Adaptation of the Sibtest Procedure

Abstract

Recently, Shealy and Stout (1993) proposed a DIF detecting procedure SIBTEST, which is 1) IRT model based, 2) non‐parametric, 3) does not require IRF estimation, 4) provides a test of significance, and 5) estimates the amount of DIF. Current versions of SIBTEST can only be used for dichotomously scored items. However, in this paper an extension to handle polytomous items is developed. This paper presents: (1) a discussion of an appropriate definition of DIF for polytomously scored items, (2) a modified SIBTEST procedure for detecting DIF for polytomous items, and (3) the results of two simulation studies comparing the modified SIBTEST with the Mantel and SMD procedures, one study with data constrained by the Rasch‐like partial credit model (same discrimination across polytomous items), and the other study with data having distinctly discrimations across items. These simulation studies indicate that the methodology of including the studied item in matching subtest for controling impact induced (group ability differences existing) Type I error tends to yield Type‐I/Type II error inflation rates that are highly unacceptable when the equal discrimination condition is violated. These simulation studies provide compelling evidence that the modified SIBTEST procedure is much more robust with regard to controlling impact‐induced Type I error rate inflation than the other procedures.

DIF Testing for Ordinal Items With Poly-SIBTEST, the Mantel and GMH Tests, and IRT-LR-DIF When the Latent Distribution Is Nonnormal for Both Groups

“…In simulations with item-varying discrimination and a nonzero mean difference (Bolt, 2002;Chang et al, 1996;Zwick et al, 1997), Type I error has been near nominal or moderately inflated for poly-SIBTEST and more accurate for poly-SIBTEST than for the Mantel test. Statistical power to detect uniform DIF increased with increases in sample size, effect size, or item discrimination.…”

Gender DIF in Reading and Mathematics Tests With Mixed Item Formats

“…The proofs for and steps of the regression correction are presented in Shealy and Stout (1993). Chang, Mazzeo, and Roussos (1996) modified the SIBTEST procedure to accommodate polytomous items when evaluating potential item DIF. First, when testing for DIF in suspect item scores, the equation:…”

Contributions to the Quantitative Assessment of Item, Test, and Score Fairness

“…As such, SIBTEST employs a measurement invariance definition of null DIF, while STAND employs a prediction invariance definition (Meredith and Millsap 1992). Chang et al (1995Chang et al ( , 1996 extended SIBTEST to handle polytomous items. Two simulation studies compared the modified SIBTEST procedure with the generalized Mantel (1963) and SMD or STAND procedures.…”