## 4 Discussion
### Implementation and Limitations of the Present Study  
When considering the findings and their practicality of the current study, design guidelines can be advised to help improve the usability of key input with two thumbs on a QWERTY soft keyboard. Note that, here, the design advice is restrained as well as cautiously made, as far as a typical layout of QWERTY soft keyboards on a smartphone is maintained. First, the key size needs to be maximized as much as the technology allows, in order to minimize the index of difficulty of a QWERTY layout and provide better vision on keys. Second, the size of the keys, especially that show relatively low touch performance and user-satisfaction (e.g., the keys that are often occluded due to the thumb have to be reached using difficult thumb motions, or need to interact with the non-dominant thumb) need to be enlarged in comparison with the other keys, in order to improve the overall user-experience in two-thumb key entry using a QWERTY soft keyboard. [PERSON] and [PERSON] [9] reported that the sizes of the keys that achieved the same goals in touch performance and user-satisfaction were different depending on the key locations in two-thumb key entry--they found that the closer the keys were located toward the periphery, the larger the sizes of the keys should become to achieve the touch performance similar to the keys near the thumbs' initial positions, and vice versa. This finding would be employed as evidence to resize the keys on a QWERTY soft keyboard. Lastly, more keys need to be assigned in the region (or close to this region) of a QWERTY layout that is supposed to interact with the dominant thumb--this is because the dominant thumb with better motor skills would be utilized as much as possible for two-thumb key entry. The present study was still in the early stage of the long research plan, and thus the research theme was narrowed down in order to ascertain the significance and validity of the study as clearly as possible. Therefore, the applicability of the study's findings and countermeasures may be tied up only with the following restricted smartphone-usecontexts: (1) two-thumb key entry with a QWERTY soft keyboard was examined using a single size of a smartphone; and (2) a single key input at once was only regarded as a text entry task. In everyday life, however, users typically send text messages with a variety of ways such as one-thumb, two-thumb, or one index finger interaction, and even use various sized smartphones. Moreover, it is certain that texting generally consists of continuous activities in which a series of letters are typed, rather than typing a single keystroke. To recap, this may be evidence enough that the findings of the present study might significantly vary depending on different smartphone-use contexts. Therefore, we strongly recommend that the findings and countermeasures of this study need to first be interpreted and applied within the smartphone-use contexts designed in the study--applications beyond the scope of this research should be implemented with caution. In addition, this study has the following weak points in the experimental protocol, and thus the findings should be cautiously interpreted and adapted. First, the thumbs' initial positions suggested in the present study were estimated using Fitts' law, and not actually measured. Although it is well known that Fitts' law is considered to be a scientifically proven tool, we need to be aware that this often leads to the invalid interpretation of the findings based on \"ground truth\" unless the initial positions are recorded via experiments. Second, the effect of the individual thumb was not investigated (i.e., which thumb predominantly taps which zone). It is certain that the findings were well matched with those of the related existing studies, but some interpretation of the results relied on conjecture based on the findings of previous mobile HCI studies.