Mobile phone touchscreens have many ergonomic problems related to text entry. Previous studies, which attempted to use ambiguous keyboards to resolve problem of small keys, focused on the disambiguation process without consideration of the user loading on graphic user interface. This study investigates user loadings, which interfere with performance in the key selection phase when using an ambiguous keyboard. Hence, three QWERTY-like ambiguous keyboards and a standard QWERTY keyboard were compared via visual search and manual operation experiments. The visual search experiment shows that layouts with many letters per key were strongly related to long visual search times, and layouts with large keys were operated faster and more accurately in the manual operation experiment. Consequently, the trade-off between visual and manual loading differed among different letter-key assignments. This study is important in that it elucidates the impacts of visual and manual loadings on ambiguous keyboards, as well as in providing user interface designers with an enhanced understanding of how to design ambiguous keyboards based on user criteria. Relevance to industry: Ambiguous keyboard designs in previous studies were generally far from the actual needs of users. This study examines the factors affecting text entry performance of users via the user-centered approach, improving the understanding of designers.
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