DETECTING DIF FOR POLYTOMOUSLY SCORED ITEMS: AN ADAPTATION OF THE SIBTEST PROCEDURE

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.