## 4 Discussion
### Comparison with Previous Studies  
In comparison with the findings of existing studies of soft keyboards on a smartphone (e.g., [8; 9; 10; 11; 22]), the following commonalities were still shared with the present study. First, task completion times significantly varied as a function of key locations. As shown in the previous studies, task completion times tended to be delayed for the keys in the periphery of the QWERTY layout in this study. It looked like touch interaction with the keys, which were likely to be visually occluded due to the thumbs or needed to be reached by using difficult thumb motions (e.g., abduction, adduction, flexion, and extension), was still a challenge in terms of speed, even for two-thumb key entry using a QWERTY soft keyboard. Second, subjective ratings were tied up with the results of task completion times. Relatively higher subjective satisfaction ratings were primarily found in the zones of the QWERTY soft keyboard where relatively faster task completion times were observed, although they did not match perfectly. It seemed that task completion times for keys still played an important role in determining user-satisfaction for key entry even using a QWERTY soft keyboard, as [PERSON] et al. [8] and [PERSON] and [PERSON] [9] claimed. Lastly, this is just conjecture but the thumbs of the dominant hands might show better touch performance than those of the non-dominant hands, especially in terms of speed. This is because relatively shorter task completion times were primarily observed for the keys on the right side of the QWERTY layout that were likely to be touched by the right thumb in this study. Given that all of the participants were right-handers in this study, we could assume that the better motor skill of the dominant hand still appeared to contribute to the improvement in touch performance in key entry even using a QWERTY soft keyboard. As aforementioned, however, this should be proven via further experiments. Simultaneously, the following unique findings were also identified from the present study. First, this study showed that the initial positions of the thumbs of both of the hands could be asymmetrically located on the left and right during two-thumb key entry. Indeed, this was expected because the present study allowed participants to hold the given smartphone in their preferred manners as usual without any guidance--this was different from the fact that the initial positions of the thumbs have been symmetrically controlled on the left and right in a large number of studies into soft keyboards on a smartphone, in order to clarify the effects of design elements in soft keyboards. We reckon that this finding suggests three meanings: (1) users may not hold a smartphone symmetrically on the left and right during texting with two thumbs on a smartphone in real life; (2) naturally, the thumbs' initial positions are likely to be asymmetrically located corresponding to the ways to hold a smartphone; and (3) touch performance during key entry could be significantly affected by these asymmetric initial positions of the thumbs. Second, the present study suggested that securing vision during key entry with a QWERTY soft keyboard might be relatively more critical than usual, in order to help improve touch performance. The study showed that task completion times and user-satisfaction were dramatically improved in the zones on the QWERTY soft keyboard (i.e., zones 2, 3, 4, 7, 8, and 9) that the participants were able to immediately see without certain effort such as moving the thumbs or changing the hand grip postures, before touch interactions--this was inconsistent with the previous studies that showed that the closer a target key is to the initial positions of the thumbs, the greater its benefit becomes in terms of touch performance (e.g., touch time, touch error, and user-satisfaction) during key entry [8; 9; 11; 22]. As aforementioned, we estimated that the reason for this inconsistency was from the fact that a large number of small keys are stuck together in a QWERTY layout.