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/Western-Digital-Ultrastar-DC-HC570-WUH722222ALE6L4-0F48155-22TB-7-2K-RPM-SATA-6Gb-s-512e-SE-3-5in-Refurbished-HDD/