Alignment as an Alternative to Anchor Purification in DIF Analyses.

Unbalanced differential item functioning (DIF) occurs when the items favoring group 1 are not balanced by items favoring group 2. This creates false differences in the total score used for matching examinee subgroups or inaccuracy in the scaling used to bring the groups to commensurable metrics. Historically, unbalanced DIF has been addressed by removing DIF items from the set of anchor items, often called purification. A recently developed alignment procedure minimizes the impact of the DIF items in scaling the groups without removing them. In this study, when the data followed a two-parameter model, the results using alignment or a purified anchor were generally comparable when the DIF was balanced or when the DIF was unbalanced but no more than 20% of the items contained DIF. With a larger percentage of unbalanced DIF, the alignment procedure yielded more accurate estimates of the group means and the magnitude of the DIF. [ABSTRACT FROM AUTHOR]