An Application of Multidimensional Vertical Scaling

Vertical scales are widely used in educational assessment as a basis for considering grade-to-grade changes in student performance. Typically, the underlying construct is assumed to be essentially unidimensional; however, if there is a change in the measured construct across grades, this assumption may be untenable. Developing a multidimensional vertical scale in these instances provides a potential solution to this problem. This paper uses empirical data from four parallel forms of a test designed to measure six foundational reading skills-administered to students in grades 6-9-to address issues in the development of a multidimensional vertical scale. The defensibility of the multidimensional structure, value-added subscores, and the stability of the scale are considered. Student growth based on unidimensional versus multidimensional estimates of ability is also presented with particular attention to implications associated with potential construct shift.