Differentiation of cognitive abilities in the WAIS-IV at the item level

It is known that studying the differentiation of cognitive abilities is associated with many methodological challenges. In the recent years, methods have been developed to address these challenges. However, these methods require that the item scores of an intelligence test are combined into a composite score which may affect the power to detect the differentiation effect or even produce spurious results. Therefore, in this study, an item level approach is presented that can be used to simultaneously test for ability differentiation, age differentiation, and age differentiation-dedifferentiation. The new method is investigated in two small simulation studies, and applied to the standardization data of the Hungarian WAIS-IV. Results indicate that the ability differentiation effect is consistently present in the items of the WAIS-IV while there is no consistent age differentiation and/or age differentiation-dedifferentiation effect