In education, psychology and health research item response theory (IRT) is increasingly used to construct tests and to evaluate the psychometric quality of existing tests. Due to the increasing availability of easy to use software, not only test developmental companies, but also researchers use IRT for the development and evaluation of tests and questionnaires. The main aim of this project is to investigate through both simulated data and empirical data, whether the main conclusions in empirical research hold under different IRT models and under different violations of IRT models. The second aim is to develop tools that can be used as effect size measures and that provide researchers practical information to what extent model choice and violations of model assumptions are important. These effect size measures will be formulated in terms of directly interpretable quantities like number of correct/incorrect decisions being made using the test.

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