DIFFERENTIAL ITEM FUNCTION

**Question:**

*How does RUMM2030 handle DIF issues and strategies?*

**Explanation:**

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

- estimating the parameters for the whole sample;
- plotting the observed means for class intervals for each group (eg. males and females) to reveal in the empirical curves whether
- there is a main effect (ICCs not crossing) or
- an interaction effect (ICCs intersecting)
- formalising the graphical display by
- conducting a two way ANOVA on the residuals
- testing an interaction effect
- testing the main effect, or both effects

**Extension:**

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:

- when the locations of the items are different but the slopes of the observed points are parallel - this is called uniform DIF and is identified by the MAIN effect for groups in ANOVA of residuals;
- when the locations are the same but the slopes are different (this is non-uniform DIF and it is detected by the INTERACTION effect in the ANOVA, OR
- both of the above.

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

- a summary of the two kinds of DIF as well as of
- the overall fit of each item to the model.

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.

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