Finally, although the ordinal logistic regression R-squared for measuring DIF in
either multicategory scores and binary scores is introduced here, it is founded on the
statistical theory for ordinal logistic regression, and the hierarchical sequential modeling
strategy implicit in the definition of DIF. Future research applying this technique to a
variety of measures will almost certainly result in a refinement of the approach and its
interpretation.
3 Note that not all R-squared measures for nominal binary responses can be
used in a hierarchical regression.