Effects of Button Width, Height, and Location on a Soft Keyboard: Task Completion Time, Error Rate, and Satisfaction in Two-Thumb Text Entry on Smartphone

This paper examined touch performance and satisfaction as a function of button width, height, and location in two-thumb interactions on a soft keyboard. Thirty-two college students with an experience of smartphone-use were recruited for testing. Task completion time (s), error rate, and subjective satisfaction were measured for different combinations of button widths (4.0, 5.3, 6.7, and 8.0 mm), heights (4.0, 5.3, 6.7, and 8.0 mm), and locations (LT: left-top, LC: left-center, LB: left-bottom, RT: right-top, RC: right-center, and RB: right-bottom). Task completion times (0.83 s) at LC and RC were significantly shorter than the peripheral button locations (0.91 s). Satisfaction scores (1: highly dissatisfied ~ 7: highly satisfied) at LB and RB (4.1) were significantly worse than the other buttons (4.8). The linear regression analyses with button widths and heights for button locations revealed that increasing button height had a relatively larger impact in improving task completion times and error rates at LT and RT than increasing button width. However, increasing button height or width rate at the other button locations had similar impacts on one another in improving the task completion times and errors.