A two-dimensional, compensatory item response model and a unidimensional model were fitted to the reading and mathematics items in the Woodcock-Johnson Psycho-Educational Battery-Revised for a sample of 1,000 adults aged 20-39 years. Multidimensional information theory predicts that if the unidimensional abilities can be represented as vectors in the two-dimensional solution, then the multidimensional model can be used to obtain ability scores with smaller standard errors. In reading, the multidimensional model yielded scores with smaller standard errors, but multidimensional scores from subtests within reading were identical to the overall reading score. In mathematics, unidimensional scores were non-linearly related to multidimensional ability estimates, and for some subjects, multidimensional ability scores had larger standard errors. There is a 16-item list of references. One table and 12 figures provide analysis information. (Author/SLD)