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Purification of the test has been a well-accepted procedure in enhancing the performance of tests for differential item functioning (DIF). As defined by Lord, purification requires reestimation of ability parameters after removing DIF items before conducting the final DIF analysis. IRTPRO 3 is a recently updated program for analyses in item response theory, with built-in DIF tests but not purification procedures. A simulation study was conducted to investigate the effect of two new methods of purification. The results suggested that one of the purification procedures showed significantly improved power and Type I error. The procedure, which can be cumbersome by hand, can be easily applied by practitioners by using the web-based program developed for this study. [ABSTRACT FROM AUTHOR]

Oshima, Raju, and Flowers demonstrated the use of an item response theory-based technique for analyzing differential item function (DIF) and differential test function for dichotomously scored data that are intended to be multidimensional. Their study assumed that the number of intended-to-be measured dimensions was correctly identified. In practice, however, the number of dimensions may be misidentified. Therefore, the purpose of this study was to demonstrate the effects of both underestimation and overestimation of the number of intended-to-be measured dimensions on the multidimensional DIF analysis using simulated two-dimensional data with known DIF items. Results show that overestimation of the number of θ traits had a consequence of decreased power. Underestimation resulted in missing a certain type of nonuniform DIF, as well as confounding the impact with DIF. Recommendations are made on how to conduct a DIF investigation with a multidimensional within-item test. [ABSTRACT FROM AUTHOR]

Two new prophecy formulas for estimating item response theory (IRT)-based reliability of a shortened or lengthened test are proposed. Some of the relationships between the two formulas, one of which is identical to the well-known Spearman-Brown prophecy formula, are examined and illustrated. The major assumptions underlying these formulas are outlined and discussed. Both prophecy formulas appear to provide comparable estimates of reliability in the IRT context as well as in the classical test theory context. [ABSTRACT FROM AUTHOR]