Optimal Touch Button Size for the use of Mobile Devices while Walking

Because of their compact design modern mobile devices facilitate ubiquitous interaction in a broad application area. Text input on the limited space of a mobile device often leads to small sizes of interaction buttons. Therefore, the commonly used keys of a soft-keyboard become very small. Hitting these keys correctly requires high-precision movements and incorrect operations are frequent. Furthermore, interaction with mobile devices often involves additional activities of the user. This context requires a specially adapted interface. It includes touch button sizes, which should be adapted or at least be adaptable to the walking condition. The goal of this study was to investigate in the optimal size of the sensitive areas of touch buttons for mobile devices. A literature research revealed that button sizes as well as interaction while walking has been addressed before, but until now the influence of walking on interaction accuracy has not been studied. Therefore, we run a study to cover this issue. Thereby, for time on task, we found highly significant differences between small button (5*5 mm) and all the other button sizes (p < 0.01). Walking had a highly significant influence for the button sizes 5*5 mm and 8*8 mm (p < 0.01). Error count showed an influence of button size (5*5 mm) compared to the other sizes (p < 0.01). Additionally, it showed a highly significant influence of walking (p < 0.01). Walking triggered a very high number of errors, especially in smaller buttons. But even in bigger buttons the error count increased significantly (p=0.041). A button size of 8*8 mm is applicable while standing, but still triggers a high number of errors. Only using button sizes of 14*14 mm showed low errors rates while walking as well as while standing. Therefore, we recommend this button size especially for designs, which are to be used while walking.