Purpose: This study aimed to examine the impact of differential item functioning in anchor items on the group invariance in test equating for different sample sizes. Within this scope, the factors chosen to investigate the group invariance in test equating were sample size, frequency of sample size of subgroups, differential form of differential item functioning (DIF), frequency of items in the anchor test with differential item functioning, directionality of differential item functioning and mean differences in subpopulation ability levels.
Research Methods: The current study was conducted by using item response theory true score equating under equivalent groups anchor test design. REMSD index was used for investigating group invariance in test equating. This study was designed as a comparison of equating results on 96 simulation conditions. The R language and SPSS software was utilized for analysis and 100 replications were performed for each condition. The effect of the conditions held in the study on group invariance in test equating was evaluated by taking average of REMSD. Also, ANOVA was performed to determine significant effect of each factor on group invariance in test equating. Findings: The findings of the study showed that differential form DIF was the factor that had the most prominent impact on group invariance in test equating.
Implications for Research and Practice: Within the scope of the results of the study, group invariance affected by factors of DIF were only in instances in which DIF in anchor items was differential across test forms.
test equating, group invariance, differential item functioning, simulation study