## 4 Discussion
### Task Completion Time, Touch Error Frequency, and Subjective Satisfaction
For instance, zones 1, 6, 11, and 12, with relatively slow task completion times, were likely to be screened by the left thumb when the thumb-tip was located in zones 2 or 13 (i.e., the left thumbs' initial positions in the current study). Simultaneously, zone 15 could be potentially obscured by the right thumb because the thumb-tip was probably placed in zones 10 or 13 (i.e., the right thumbs' initial positions in the current study) before touch interaction. Likewise, the zones (mostly in the center area of the keyboard) that were not likely to be visually occluded due to the thumbs' initial positions showed relatively shorter task completion times. [PERSON] and [PERSON] [9] explained these phenomena that visually occluded keys could delay touch interaction during key entry on a smartphone by the fact that users might need to unnecessarily do one more step called searching a target with the eyes before moving their fingers. Second, the zones that had to be reached by flexing the interphalangeal and metacarpophalangeal joints of the thumbs were well-matched with the zones with relatively longer task completion times in the present study. For example, zones 6, 11, and 12 that could be reached by flexing the left thumb from the initial position were roughly accorded with the zones that showed relatively slow task completion times on the left side of the keyboard (zones 1, 6, 11, and 12). In the same manner, zone 15, which could be reached by flexing the right thumb, was completely consistent with the zones that showed relatively slow task completion times on the right side of the keyboard (zone 15). In fact, this was expected because the flexion of the thumbs typically requires a great deal of perceived effort as well as a large amount of joint displacement [8; 9; 22; 23; 24; 25]. Therefore, we believed that it was natural that touch interactions were delayed in zones 6, 11, 12, and 15 during key entry. All things considered, to recap, these findings could be regarded as evidence enough to support the notion that the patterns in the task completion times were strongly affected by the thumbs' initial positions in the present study. On the other hand, overall, the task completion times on the left side of the keyboard were longer than those on the right side. We estimated that this was related to the fact that all of the participants recruited for the present study were right-handers, since it is well known that the dominant hands typically outperform the non-dominant hands in most motor tasks, especially in speed and accuracy [26; 27; 28], i.e., this is just a guess but this might indicate that the thumb of the dominant hand still showed more superior touch performance than that of the non-dominant hand in key entry, even using a QWERTY soft keyboard. However, this should be verified via experimental methods. Touch error frequencies failed to show statistical significance among the zones. In other words, this indicated that touch errors may have occurred, regardless of the locations of keys on the QWERTY soft keyboard in the present study. However, we assumed that this might be a premature judgement, in considering the fact that each experimental trial was only repeated twice during testing. Note that, indeed, the number of touch errors was too small in the present study as well, perhaps because the size of the keys (0.5 cm \(\times\) 0.7 cm) employed in the present study was used in a large number of smartphones often enough that the participants could feel familiarity with them. This was because the lack of experimental trials could have an adverse effect on the statistical significance of the touch error frequencies among the zones, i.e., an adequate number of touch errors to have statistical power may not have been acquired in this study. As evidence, although not statistically significant, faint but reasonable patterns in the touch error frequencies were observed as a function of the zones in this study.