system (human, robotic, VE) efficiently. Ideally, this set of basis tasks should be such that (1) knowledge of performance on these tasks enables one to predict performance on all tasks of interest and (2) it is the minimal set of tasks (in terms of time consumed to measure performance on all tasks in the set) that has this predictive power.

Two basic psychophysical questions in evaluation are: (1) With a given set-up, how good is the task performance or realism of the subjective experience? (2) How does a change in the set-up improve the performance of a given task, realism of the experience, or both? An example of the former is the investigation of the consequences of using an ungrounded display to simulate contact forces that really stem from grounded sources. In the latter question, the word change is to be interpreted in a broad sense and includes modifications of the interface hardware, object models, interaction software, and addition/subtraction of visual or auditory modalities. Theoretical and experimental approaches to quantify information transfer rates to and from the user under various single and multimodal conditions need to be developed.