The purpose of this study was to investigate whether different grip spans affected the touch performance of one-handed thumb interaction with a smartphone. Tapping time, hit count, and thumb-tip displacement were measured while using three mock-ups of an iPhone 5S, 6, and 6 Plus, respectively. In the tapping task, four positions were tested: top-left, top-right, bottom-left, and bottom-right. The results showed that the hit count (F2,14=8.596, p=.004) and thumb-tip displacement (F2,14=11.348, p=.001) significantly decreased as the grip span increased. Thus, we concluded that there existed another problem beyond the limit of thumb length that caused a reachability issue when using a large smartphone. Thus, this should be considered in the user interface design of a large smartphone.
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