Most acceptable and least capable target sizes for zoom-and-click tasks on mobile touchscreens- implications for user performance and applications to interface design

Abstract

Using mobile devices with small touchscreens to click on icons or links was problematic for users, especially when the layout was dense and full of targets. Most of existing literatures studied proper target sizes in clicking tasks by testing user performance with predetermined options of targets. Larger targets were suggested for elders based on limited number of alternatives in discrete sizes. However, multi-touch technology now allows users to manipulate target sizes by zooming with a pinch gesture in a continuous manner. A speed-accuracy balanced target size (the Most Acceptable Target Size, MATS), i.e. a target not only large enough to ensure accuracy in clicking but also small enough to avoid unnecessary operation in zooming, becomes more relevant in this context. The current study recorded the MATS determined subjectively by young and elder users in a zoom-and-click task on mobile devices using 5-, 7- and 10-inch touchscreens with four levels of display sparsity. User performance was measured together with the Least Capable Target Size (LCTS, the target just large enough for the user to click with above-chance accuracy) indicative of users’ capability. Results showed that young and elder users preferred similar MATS (7.42 mm on average). The 10-inch device required the smallest LCTS but the largest MATS due to greater viewing distance and the unfamiliar 7-inch device required the largest LCTS. Reduction of display density can cause larger MATS required and possibly elicit speed-accuracy trade-offs for elders to adapt.