Background and objective: As cognitive, motor, and sensory skills decline with age, the interface needs of elderly users differ from that of young adults, especially when entering important information in healthcare applications (e.g. blood glucose values) via a numeric keypad on a smartphone touchscreen. The aim of this study is to propose an optimal numeric keypad design for elderly users. A total of 51 participants greater than or equal to 65 years old completed a 6-digit numeric entry task on a smartphone touchscreen. The Java-based experimental program allows participants to operate 45 different keypad designs consisting of three button shapes (circle, vertical rectangle, and square), five button sizes with 2.5-mm increments (7.5 mm, 10 mm, 12.5 mm, 15 mm, and 17.5 mm), and three button spacings (0 mm, 1 mm, and 3 mm). The usability of the keypad design was compared based on task completion time, error rate, and subjective satisfaction, while the data was analyzed using repeated measures ANOVA. The results show that circle and square buttons outperform the vertical rectangle button in completion time, accuracy, and satisfaction. Square and circle buttons in 15 mm size led to improved efficiency and decreased error rate. Moreover, task completion time for 3 mm is significantly shorter than 0 mm spacing. Given sufficient space on the touchscreen, the 3 mm spacing can be considered optimal when designing a numeric keypad interface. This study presents a comparison of different numeric keypad designs and the impact on the performance and satisfaction of elderly users. The result can be applied to smartphone numeric keypad design to increase input accuracy and speed. In addition, both circle and square buttons can be adopted in user interfaces for elderly users without the decline of usability. Relevance to industry: The proposed configuration of button shape, size, and spacing can be applied to the numeric keypad user interface (UI) of smartphones targeting elderly users, especially in healthcare applications.
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