Standard errors and bias of unidimensional and multidimensional ability estimates were compared in a factorial, simulation design with two item response theory (IRT) approaches, two levels of test correlation (0.42 and 0.63), two sample sizes (500 and 1,000), and a hierarchical test content structure. Bias and standard errors of subtest scores (subtests within longer tests) were smaller for the multidimensional approach, but the bias of longer test scores did not improve. Sample size and test intercorrelation had a negligible effect on the results. As the number of items in a test becomes large, in an absolute sense, and relative to the total number in the battery, the advantage of the multidimensional approach seems to disappear. Two tables and 13 figures present results of the simulation. There is an 11-item list of references. (SLD)