4.1. Task Completion Time, Touch Error Frequency, and Subjective Satisfaction
As expected, the task completion times varied significantly by zones on the QWERTY soft keyboard. The results showed a couple of unique patterns that were somewhat different from the previous studies, which showed two findings for key entry using the thumbs [
8,
9,
11,
22]: (1) the shortest task completion times were found near the center area of the keyboard (i.e., approximately around zone 7 for the left thumb and zone 9 for the right thumb in this study), where the thumbs were designated to stay before every movement for touch interaction (i.e., called the initial positions of the thumbs) for testing, and (2) the farther away from the initial positions of the thumbs the target keys were, the longer the task completion times became. First of all, the shortest task completion times were observed in zones 2, 10, and 13 in the present study. When considering the finding of the previous studies that the shortest touch times were observed around the thumbs’ initial positions according to Fitts’ law, this can be interpreted as the fact that the initial positions of the thumbs in the present study were likely to be located near those zones. That is, in this study, the initial positions of the thumbs were placed asymmetrically around zones 2 and 13 for the left thumb and zones 10 and 13 for the right thumb. It was unclear why the initial positions of the thumbs were found in these zones in the present study. However, we estimated that the habits of grasping a smartphone with the hands would influence this phenomenon, because participants were allowed to grasp the given smartphone in their preferred ways as usual in the present study. To give a plausible example, for two-thumb text entry using a QWERTY soft keyboard, users tend to grasp a smartphone (in particular, its lower part) with their dominant hand first and then wrap it using the other hand. Naturally, this could help the tip of the dominant thumb (i.e., the thumb-tip of the right hand in this study) to be located near the right border of the QWERTY keyboard section (approximately near zones 5 and 10 in the present study) because the back of the phone is likely to rest on the palm of the dominant hand—in particular, the deeper the smartphone is held in the palm of the dominant hand, the clearer this tendency could become. In addition, the tip of the non-dominant thumb (i.e., the thumb-tip of the left hand) may be located toward the center area of the keyboard (approximately near zones 2, 7, 8, 12, and 13 in the present study), since the non-dominant hand needs to grasp the smartphone over the dominant hand. However, this mechanism is one assumption based on the participants’ debriefing session of the present study, and thus further study is warranted to verify this habitual way to grasp a smartphone and its relationship with the thumbs’ initial positions for text entry.
In addition, the zones with significantly longer task completion times were more prevalently observed in the bottom row and the very left column of the zones—this was also a little different from the aforementioned studies that showed the regular pattern that task completion times gradually increased as a target key moved from the center (i.e., the thumbs’ initial positions) to the periphery areas on the QWERTY keyboard. We estimated that this propensity was strongly associated with the thumbs’ initial positions that were asymmetrically found in the present study. A couple of pieces of evidence support this assumption. First, the zones with relatively longer task completion times were roughly matched with the keys on the keyboard that could be visually occluded due to the thumbs’ initial positions. 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. Chang and Jung [
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 × 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. 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.
4.2. 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 Chang et al. [
8] and Chang and Jung [
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. This is because users not only had to face more options despite the benefit of muscle memory from the familiarity with a QWERTY layout but also made greater cognitive and physical efforts to accurately locate a small target among these options. Naturally, it was assumed that securing vision would become relatively more useful than usual for improving task completion time and user-satisfaction during key entry using the QWERTY soft keyboard.
Lastly, it seemed that Fitts’ law worked subordinately during key entry in this study. This was because if Fitts’ law played a primary role in key entry, task completion times should have been gradually increased as the target keys moved away from the thumbs’ initial positions. Of course, these propensities were essentially observed near the zones that were considered to be the thumbs’ initial positions in this study. Simultaneously, however, several zones (e.g., zones 1, 6, 12, and 15) tended to show disproportional delay in their task completion times even though they were located right next to the zones that were regarded as the initial positions of the thumbs. We expected that this would suggest two meanings. First, the distances between keys and the initial positions of the thumbs would not be a primary factor to determine task completion times in key entry using a QWERTY soft keyboard. Note that we assumed that the index of difficulty only depended on the distances in Fitts’ law, because all of the keys employed in this study shared the same size. Second, it looked like Fitts’ law still worked fundamentally in determining task completion times for keys during key entry using a QWERTY soft keyboard, but task completion times for keys might have been more significantly affected by other factors. In fact, as aforementioned, we assume that securing vision on keys and physical effort to reach keys might be regarded as the potential candidates of those other facts that have more impacts on key entry using a QWERTY soft keyboard. In sum, these suggestions should be cautiously made. When considering that each zone employed in the present study was composed of two or three keys, the patterns in task completion times would be different in the level of the keys. Therefore, further study is needed for verifying this. Note that, in fact, we already found similar propensities even from the key level after the raw data on the keys in this study were reviewed.
4.3. 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 vison 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. Chang and Jung [
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-use contexts: (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. 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.