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The quality of user interfaces is often measured in terms of efficiency, effectiveness, and satisfaction. In the area of tangible user interfaces, epistemic-or exploratory-action has been suggested as a fourth measure of quality. In computer game studies (Kirsh & Maglio, 1992, 1994), players used epistemic actions to modify the environment, which helped them determine the correct position of blocks with less mental effort. There, the researchers found that it might be easier to physically modify the external world and then interpret it than to compute and interpret a new state mentally. Specifically, epistemic action may be a relevant concept when researching tangible user interfaces incorporating physical handles. This article examines the potential relations between the three traditional measures of usability and epistemic actions using three spatial planning tools with different degrees of physicality. The results indicate that epistemic action is a measure that is independent of the three traditional usability measures: efficiency, effectiveness, and satisfaction. However, epistemic action does not increase linearly with the physicality of a user interface, and it probably is a more complex measure that is also related to the reusability of the interface. Further research is needed to fully understand the potential of this measure. [PUBLICATION ABSTRACT]