How does RUMM2030 handle DIF issues and strategies?


When conducting a Rasch analysis, RUMM2030 carries out a DIF analysis using the following sequence:


There are many ways of checking for DIF, but the key is that the item works the same way for two or more different groups, irrespective of their locations on the trait. Because we do not have the actual locations, these generally have to be estimated. There are three basic forms of systematic DIF:

The MAIN effect for class intervals in RUMM2030 tests for the overall fit, irrespective of the groups into which people are classified. Thus the ANOVA of residuals gives

It is possible that there is an overall fit of an item when groups are not considered, even though DIF is observed when persons are classified into groups, and vice versa.