DIF Cancellation in the Rasch Model.

Differential item functioning (DIF) cancellation occurs when the cumulative effect of an item or set of items exhibiting DIF against one subgroup cancels with other items that exhibit DIF against the comparison group and hence results in non-existent DIF at the test level. This paper investigates DIF cancellation in the context of Rasch measurement. It is shown that this phenomenon is not a property of the Rasch model, but rather, a function of the manner in which item parameters are estimated and the way that DIF impacts these estimates. The conditions under which DIF cancellation would exist when using the Rasch model are suggested and a proof is provided to support this suggestion. Empirical examples are provided to refute prior suggestions that DIF cancellation always exists if the Rasch model is used.