DIMTEST is a statistical test procedure for assessing essential unidimensionality of binary test item responses. The test statistic T used for testing the null hypothesis of essential unidimensionality is a nonparametric statistic. That is, there is no particular parametric distribution assumed for the underlying ability distribution or for the item characteristic curves generating item response in the mathematical derivation of probability distribution of the statistic T. The purpose of the present study is to empirically investigate the robustness of the statistic T with respect to ability distributions. Several nonnormal distributions, both symmetric and nonsymmetric, are considered in simulations involving six different types of ability distributions. In addition, test length and sample size are used as parameters in the present study. Simulation results indicate that the performance of Stout's statistics T subscript c and T subscript p are consistent with their theoretical developments, in that no particular shape is assumed for examinee abilities. That is, these statistics are robust against the shape of the ability distribution. Included are seven tables. (Contains 8 references.) (Author/SLD)