Modeling multidimensional test data with a unidimensional model can result in serious statistical errors, such as bias in item parameter estimates. Many methods exist for assessing the dimensionality of a test. The current study focused on DIMTEST. Using simulated data, the effects of sample size splitting for use with the ATFIND procedure for empirically deriving a subtest composed of items that potentially measure a second dimension versus DIMTEST for assessing whether this subtest represents a second dimension were investigated. Conditions explored included proportion of sample used for ATFIND, sample size, test length, interability correlations, test structure, and distribution of item difficulties. Overall, it appears that DIMTEST has Type I error rates near the nominal rate and good power in detecting multidimensionality, although Type I error inflation is observed for larger sample sizes. Results suggest that a 50/50 split maximizes power and keeps the Type I error rate below the nominal level unless the test is short and the sample is large. A 75/25 split controls Type I error better for short tests and large samples. (Contains 4 figures and 1 note.)