The qualitative characterization of individual performance that is central to modern psychological theory is not adequately modeled by traditional psycho-metric theory that assumes, among other things, unidimensionality. In the present study, data are presented that are more adequately modeled by HYBRID, a model that incorporates both latent trait and latent class components. The latent classes were defined by a cognitive analysis of the understanding that individuals have for a circumscribed domain. In addition to providing a better statistical fit, the analysis also improves the amount of diagnostic information available for a given individual. (36pp.)
