A large-scale experimental study was conducted involving the training and testing of 123 human subjects on the Criterion Task Set (Version 1.0). Testing was performed under baseline and stressor conditions. The performance data and Subjective Workload Assessment Technique ratings for the first baseline trial (Trial 6) were analyzed using the SAS VARCLUS procedure to evaluate the structure of the CTS. Seven clusters of response time variables were identified for the nine tasks. In general, the Memory Search, Linguistic Processing and Mathematical Processing tasks were grouped in one cluster with each of the other clusters representing a single task. Five clusters were identified for the SWAT ratings with clusters differentiated along the dimensions of task difficulty and processing stage.