Group Differences Based on Irt Scores: does the Model Matter?

In this study, effect sizes based on simulated groups were compared for the one-parameter and three-parameter logistic IRT models. Data were generated based on a three-parameter model, and item estimates were obtained from the simulated data based on both the one-parameter and three-parameter models. Abilities were estimated using both maximum likelihood and expected a posterior methods. The data fit the three-parameter model much better, but there were only minimal differences between the effect sizes based on different models.