Investigating Measurement Invariance in Computer-Based Personality Testing: The Impact of Using Anchor Items on Effect Size Indices.

A popular method to assess measurement invariance of a particular item is based on likelihood ratio tests with all other items as anchor items. The results of this method are often only reported in terms of statistical significance, and researchers proposed different methods to empirically select anchor items. It is unclear, however, how many anchor items should be selected and which method will provide the "best" results using empirical data. In the present study, we examined the impact of using different numbers of anchor items on effect size indices when investigating measurement invariance on a personality questionnaire in two different assessment situations. Results suggested that the effect size indices were not influenced by using different numbers of anchor items. The values were comparable across different number of anchor items used and were small, which indicate that the effect of differential functioning at the item and test level is very small if not negligible. Practical implications are discussed and we discuss the use of anchor items and effect size indices in practice.