The number of extraneous touches, or number of user touches on a Touch Screen Display (TSD) that were in excess of the minimum required to perform a procedural task, was investigated as an implicit usability metric during a usability study. Data for this pilot study were obtained from a usability assessment performed to evaluate old and new versions of a TSD for adjusting system configuration settings on a system for the US Navy. Participants (N = 28) performed a training mission on a gunnery trainer, during which they engaged simulated enemy boats while being interrupted to perform procedural tasks on a TSD at regular intervals. The count of extraneous touches was significantly correlated with task completion time and System Usability Scale (SUS) scores, providing notional support for its use as a usability metric. The advantages of this metric and limitations of these findings are discussed.
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