IRT DIFFERENTIAL ITEM FUNCTIONING - AN EXAMINATION OF ABILITY SCALE PURIFICATIONS

Item Response Theory (IRT) differential item functioning (DIF) methods were employed to determine the accuracy of item classification as bia sed or unbiased. The procedure involved a combination of Drasgow's ite rative item parameter linking method and Lord's ability scale purifica tion procedure. Biased items were created using a two-dimensional nonc ompensatory IRT model. Previous research had demonstrated that the ite rative linking and ability scale purification (ILAP) method held promi se for discerning biased from unbiased items in one simulated unidirec tional DIF condition. The present study found that ILAP was at times m ore effective than iterative linking alone primarily by reducing false negatives and occasionally reducing false positive misidentifications . The choice of significance level employed for detection of DIF did i nfluence the convergence of the ILAP method. The method does hold some promise for improving correct classification of test items as biased or unbiased.