## 4 Discussion
### Implementation and Limitations of the Present Study
Third, the participants may have had difficulty differentiating the 26 letter keys when rating the subjective satisfaction in the present study. Although they were allowed to freely manipulate the smartphone with no time limit for the deliberate evaluation, it might have been challenging for them to logically compare the touches of 26 similar items. Lastly, more participants needed to be recruited for testing, and organized debriefing sessions for the participants were necessary. The study showed some evidence that more participants were required. For example, if a sufficient number of touch errors had been acquired, the patterns in touch errors would have become apparent enough to be statistically significant. In addition, since the debriefing sessions were conducted only with the volunteers among the participants, their opinions were not collected systematically enough to suggest specific ideas. Further research is warranted for addressing the knowledge gap in the present study and helping to generalize the findings. First, studies involving different age groups are needed. The current study only employed college students in their 20s as participants, since they were considered to be major smartphone users [34; 35]. However, the researchers emphasize that tactile performance varies significantly with age [36; 37]. Therefore, research including different age groups could be useful in helping to generalize the findings of this study. Second, left-handed people should be included in testing to examine the effect of handedness during two-thumb key entry. In the current study, only right-handers were recruited in order to minimize unwanted factors in the experiments and clearly observe the effects of independent variables. However, as is well known, handedness is regarded as a significant factor affecting hand motor skills, especially in motion speed and accuracy [26; 27; 28]. We believe that further studies including left-handers could help to not only investigate the effects of the non-dominant hand during two-thumb key entry but also to generalize the findings of this study. Third, increasing the number of experimental trials could be helpful in clarifying touch error patterns that occur during key entry using a QWERTY soft keyboard. Although not statistically significant, the present study found some faint but reasonable patterns in touch errors. It looked like these patterns could become apparent enough to be statistically significant if the number of touch errors was sufficiently acquired, i.e., increasing the repetition of experimental trials may be helpful in compensating for the lack of statistical power in the touch errors and furthermore contribute to clarifying this issue. Lastly, obtaining the positions of touch errors would be useful for understanding the patterns in touch errors while using a QWERTY soft keyboard. Since the present study only focused on the number of targets missed, there was a restrictive approach to interpreting the patterns in touch errors and identifying their causes. It is well known that touch error coordinates typically provide more insight in terms of user-experience (e.g., which zone has a small or large touch offset of touch errors), because they include geographic information such as the distance and direction from targets [10]. Thus, obtaining touch error coordinates is recommended in order to obtain more valuable and scientific hints for improving the user-experience of a QWERTY soft keyboard on a smartphone.