This short paper introduces an enhancement to the Keystroke-Level Model (KLM) by extending it with three new operators to describe interactions on mobile touchscreen devices. Based on Fitts’s Law we modelled a performance measure estimate equation for each common touch screen interaction. Three prototypes were developed to serve as a test environment in which to validate Fitts’s equations and estimate the parameters for these interactions. A total of 3090 observations were made with a total of 51 users. While the studies confirmed each interaction fitted well to Fitts’s Law for most interactions, it was noticed that Fitts’s Law does not fit well for interactions with an Index of Difficulty exceeding 4 bits, highlighting a possible maximum comfortable stretch. Based on results, the following approximate movement times for KLM are suggested: 70ms for a short untargeted swipe, 200ms for a half-screen sized zoom, and 80ms for an icon pointing from a home position. These results could be used by developers of mobile phone and tablet applications to describe tasks as a sequence of the operators used and to predict user interaction times prior to creating prototypes.
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