Differential Item Functioning

Differential item functioning, known as DIF, is a statistical property of a test item describing how likely individuals from different groups, even when they possess comparable underlying ability, are to answer that item differently. Uniform DIF gives one group a consistent advantage across all ability levels, while nonuniform DIF means the size of the advantage changes depending on the test taker's own ability level. Detecting DIF does not by itself prove that an item is biased; it flags unexpected item behavior that then requires review and judgment. Test developers commonly identify DIF using the Mantel-Haenszel procedure, logistic regression, item response theory based methods, and confirmatory factor analysis.