Estimating tests including subtests

Current assessment studies often face a dilemma that arises from the necessity of a simultaneous unidimensional and multidimensional interpretation of a test. When, additional to the measurement of a single domain, subdomains of this domain are to be measured, one and the same data set will have to be analyzed unidimensionaly and multidimensionaly at the same time even though it is a contradiction of the theoretical assumptions underlying the analysis. This article, at first, describes the psychometric deficiencies typically going along with current analysis approaches. Subsequently, a subdimension model is proposed that explicitly allows for the existence of subdomains, or subdimensions, in a measured domain. Thereby, the model provides a means to obtain calibration results that suffer less from the deficiencies described and also allows for an item selection in the test development process that already considers the multidimensional structure of the test.