Quantifying Touch: New Metrics for Characterizing What Happens During a Touch

Measures of human performance for touch-based systems have focused mainly on overall metrics like touch accuracy and target acquisition speed. But touches are not atomic—they unfold over time and space, especially for users with limited fine motor function, for whom it can be difficult to perform quick, accurate touches. To gain insight into what happens during a touch, we offer 15 target-agnostic touch metrics, most of which have not been mathematically formalized in the literature. They are touch direction, variability, drift, duration, extent, absolute/signed area change, area variability, area deviation, area extent, absolute/signed angle change, angle variability, angle deviation, and angle extent. These metrics regard a touch as a time series of ovals instead of a mere (x, y) coordinate. We provide mathematical definitions and visual depictions of our metrics, and consider policies for calculating our metrics when multiple fingers perform coincident touches. To exercise our metrics, we collected touch data from 27 participants, 15 of whom reported having limited fine motor function. Our results show that our metrics effectively characterize touch behaviors including fine-motor challenges. Our metrics can be useful for both understanding users and for evaluating touch-based systems to inform their design.