Instrument dimensionality. Instruments are typically designed to capture one or
more attributes (or ‘traits’) about the knowledge, performance, or attitudes held by a
sample. Quite often, different item sets are used to measure different traits (or
aspects of a single trait). These aspects may be referred to as instrument ‘dimensions.’
Particular assemblages of items in an instrument are typically used to
measure these different dimensions. In theory, an individual Rasch analysis may be used to measure each of these dimensions; however, a multi-dimensional Rasch
analysis permits analysis of the dimensionality of a given set of items. In other words,
Rasch analysis may be used to determine empirically whether a set of items is in fact
measuring two or more different traits. If there is a theoretical justification for
assuming multi-dimensionality for a set of items (i.e., the instrument was designed
to measure several traits), then a multi-dimensional as well as a uni-dimensional
Rasch model may be employed to examine instrument structure.