A new method for estimating differential item functioning (DIF) for multiple groups and polytomous items: Development of index K and the computer program "EasyDIF"

We propose a new method, called index K, to estimate the degree of differential item functioning (DIF) on a questionnaire or test taken by multiple groups. A computer program (EasyDIF) was developed to calculate index K. In developing the program, the three following concerns were paramount: (a) the program is applicable when there are more than three groups to be analyzed, (b) it is able to accommodate polytomous items, such as a Likert scale, and (c) details of DIF can be presented in a graph. In simulations and using Big Five scale data, the performance of index K was comparable to the Mantel-Haenszel method and to the PoIy-SIBTEST. It is necessary to repeat the calculations to obtain index K. However, because EasyDIF has a graphical user interface, it was intuitively easy to use. Therefore, the results were easier to obtain using EasyDIF.