Differentiation of cognitive abilities occurs if the structure of intelligence changes across a given individual differences variable. Past studies have focused on establishing differentiation with respect to general intelligence, or ‘ability differentiation’, and differentiation with respect to age, or ‘age differentiation’ (Spearman, 1927). Typically, these studies focused on differences in the covariance structure of intelligence subtest data (i.e., sum scores or factor scores) across groups that differ in age or ability (see e.g., Molenaar, Dolan, Wicherts, & Van der Maas, 2009 for an overview). Results are however rather mixed, showing different effects between studies or even within studies. An important aspect about these mixed results is that the differentiation effect is in essence an interaction effect (Tucker-Drob, 2009) which makes it challenging to study. That is, as interaction effects are scale dependent (e.g., Loftus, 1978), differentiation effects may show up differently in different subtests due to arbitrary properties of the data (e.g., difficulty of the items, number of the items, scale of the items). A possible solution to this problem is to consider the differentiation effect at the item level instead of the subtest level. Such item level analyses take the characteristics of the items into account such that the results do not depend on arbitrary properties of the data. In the present talk, a possible item level approach to test for ability differentiation is presented and applied to the item level data of the Hungarian standardization data of the WAIS-IV which covers respondents between 16 to 19 years of age.
References
Loftus, G. R. (1978). On interpretation of interactions. Memory & Cognition, 6(3), 312-319.
Molenaar, D., Dolan, C.V., Wicherts, J.M., & van der Maas, H.L.J. (2010). Modeling Differentiation of Cognitive Abilities within the Higher-Order Factor Model using Moderated Factor Analysis. Intelligence, 38, 611-624.
Spearman, C. E. (1927). The abilities of man: Their nature and measurement. New York: Macmillan.
Tucker-Drob, E. M. (2009). Differentiation of cognitive abilities across the life span. Developmental Psychology, 45, 1097–1118.
