ABSTRACT
Much of item response theory (IRT) is based on the assumption of unidimensionalty; namely, the associations among the item responses are explained completely by a single underlying latent variable, representing the target construct being measured. While this is often justified in many areas of educational measurement, more recent interest in measuring patient-reported outcomes (Gibbons et al., 2008, 2012) involves items that are drawn from multiple uniquely correlated subdomains violating the usual conditional independence assumption inherent in unidimensional IRT models.
