## 4 Discussion
### Task Completion Time, Touch Error Frequency, and Subjective Satisfaction
For example, touch errors were more prevalent on the left side of the keyboard--as aforementioned, this might have been affected by the fact that the dominant hand usually has better motor performance than the non-dominant hand in speed and accuracy [26; 27; 28]. In addition, relatively more touch errors were found, especially in the zones located in the periphery area of the keyboard--this was natural because the keys in these zones typically inflated the index of difficulty according to Fitts' law and thus more conscious muscle control and perceived effort could be required for reaching such keys [9; 10; 29; 30]. We estimated that these phenomena would have become apparent enough to be statistically significant if the number of touch errors was sufficiently acquired in the present study. That is, in conclusion, further research is needed to identify the significant relationships between the touch error frequencies and the locations of keys on the QWERTY soft keyboard. In particular, increasing the number of experimental trials could be useful for clarifying this issue. Relatively higher subjective satisfaction ratings were found in the center area of the QWERTY layout, including zones 2, 3, 4, 7, 8, and 9. It looked like the subjective satisfaction ratings were roughly tied up with the task completion times, as previous studies have claimed [8; 9; 11], although there were differences in zones 5, 10, and 13. However, simultaneously, the pattern of subjective satisfaction ratings found in this study was somewhat different from the finding of the studies that the keys near the thumbs' initial positions (with the shortest task completion times) showed the highest subjective satisfaction ratings for key entry. We estimated that these could be induced by the differences in the soft keyboard layouts employed for testing--that is, the present study employed a typical QWERTY soft keyboard consisting of 26 alphabet letter keys that were stuck together next to one another in the keyboard section (i.e., a small space), while those studies used only part of the keyboard, or a few of the keys were placed far apart from one another. In sum, relatively more cognitive and physical efforts could be required during key entry while using a soft keyboard with more than 26 keys, since users need to locate targets out of many alternatives [14; 15]. In fact, since muscle memory (from familiarity with a QWERTY layout) could help the thumbs to move close to the target keys, the number of alternatives that users' thumbs face is probably reduced to three to six keys, including the surrounding and target keys. Nevertheless, it is inferred that eyes-free finger-based touch attributed to muscle memory is regarded as conceivably inaccurate on small keys (4 to 6 mm) [31; 32; 33], and thus users still need to make more efforts to avoid typos. Naturally, this might be interpreted as the fact that securing vision on target keys would have a relatively greater impact than usual in key entry while using a QWERTY soft keyboard, and furthermore it could significantly affect users' subjective satisfaction--this mechanism was similar to the reason why the zones with less opportunities of being visually occluded by the thumbs showed relatively shorter task completion times in this study. To recap, this might be a plausible reason to explain the high satisfaction ratings of zones 2, 3, 4, 7, 8, and 9 during key entry--this was also supported by the opinions of the participants in the debriefing session of the present study. On the other hand, it was still unclear why zones 5, 10, and 13 showed relatively lower subjective satisfaction ratings despite their relatively shorter task completion times. However, we assumed that this phenomenon was strongly relevant to the fact that zones 5, 10, and 13 were located in the areas that could be relatively easily occluded due to the thumbs in the field of view, as well as the fact that they needed to be reached by using relatively difficult thumb motions such as abduction/adduction and flexion/extension.