Unidimensional item response theory (IRT) models are useful when each item is designed to measure some facet of a unified latent trait. In practical applications. items are not necessarily measuring the same underlying trait. and hence the more general multi-unidimensional model should be considered. https://www.roneverhart.com/MagnaFlow-2003-2006-Toyota-Corolla-Street-Series-Cat-Back-Performance-Exhaust-System/