Differential Item Functioning - Forms of DIF

Forms of DIF

Uniform DIF is the simplest type of DIF where the magnitude of conditional dependency is relatively invariant across the latent trait continuum (θ). The item of interest consistently gives one group an advantage across all levels of ability θ. Within an item response theory (IRT) framework this would be evidenced when both item characteristic curves (ICC) are equally discriminating yet exhibit differences in the difficulty parameters (i.e., a_r = a_f and b_r < b_f) as depicted in Figure 1. However, nonuniform DIF presents an interesting case. Rather than a consistent advantage being given to the reference group across the ability continuum, the conditional dependency moves and changes direction at different locations on the θ continuum. For instance, an item may give the reference group a minor advantage at the lower end of the continuum while a major advantage at the higher end. Also, unlike uniform DIF, an item can simultaneously vary in discrimination for the two groups while also varying in difficulty (i.e., a_r ≠ a_f and b_r < b_f). Even more complex is “crossing” nonuniform DIF. As demonstrated in Figure 2, this occurs when an item gives an advantage to a reference group at one end of the θ continuum while favors the focal group at the other end. Differences in ICCs indicate that examinees from the two groups with identical ability levels have unequal probabilities of correctly responding to an item. When the curves are different but do not intersect, this is evidence of uniform DIF. However, if the ICCs cross at any point along the θ scale, there is evidence of nonuniform DIF.