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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.
In the field of human-computer interaction, the usability of a program or tool is often measured in terms of efficiency, effectiveness, and satisfaction (ISO/IEC, 1998).1 Recently, in the field of tangible user interfaces (TUIs), an additional measure for the quality of tangible tools was introduced: epistemic action. Kirsh and Maglio (1992, 1994) distinguished between observable user actions as being either "epistemic" or "pragmatic." An epistemic action is an action whereby users change their environment to search for a solution or strategy to perform a certain task rather than to move closer to an external goal state. A pragmatic action is strictly the action needed to perform this task. Epistemic action is sometimes also called "trialand- error" (Sharlin, Watson, Kitamura, Kishino, & Itoh, 2004) or "exploratory motor activity." Kirsh and Maglio (1992) illustrated epistemic action with the example of how players of the computer game Tetris rapidly rotate falling bricks instead of mentally determining the correct position for a brick and then rotating it to that position. Players use epistemic actions to modify the environment,...