1. Introduction
Smartphones have played an essential role as communication devices [
1]. People perceive a smartphone as a necessity in everyday life and for work because of its various functionalities, including Internet service [
2]. The penetration rate of smartphones has exceeded 50% in many western countries, and more than 80% of mobile phone users use smartphones in South Korea [
3]. Because smartphone penetration is increasing and the market is becoming saturated, companies are making great efforts to satisfy not only technological progress but also user’s perceived emotion [
4].
In order to gain a competitive edge over competitors in a highly competitive market, research has been actively undertaken to determine the relationship between various dimensions related to the design of smartphones, such as length, width, thickness, and weight, and the user’s subjective feelings. Pereira et al. [
5] studied the relationships between subjective fatigue ratings and the physical features of smartphones, including size, weight, shape, and material. Lee et al. [
6] identified optimal reach zones for the back-side interaction on smartphones with different widths and lengths based on subjective discomfort and muscle activation data when various tasks were given. Sung et al. [
7] investigated the touch performance of one-handed thumb interactions with iPhone 5S, 6, and 6 Plus by measuring the tapping time, hit count, and thumb-tip and determined reachability issues for large smartphones. In addition, Chowdhury and Kanetkar [
8] determined the preferred size of smartphones with hand anthropometry data and in terms of smartphone handiness.
With the increase of information provided by smartphones and the development of technologies related to flexible displays, smartphones including curved displays are being launched. This type of display is called “edge”, which means a curved display implemented on one or both sides [
9]. In fact, the Galaxy Note Edge was launched as the first smartphone with an edge screen in 2014. Since then, the Galaxy Edge series has steadily been released. Moreover, other major manufacturers also started to launch smartphones with edge screens, such as the Xiaomi Note 2 and Vivo Xplay 6.
From the viewpoint that design variables such as size, shape, etc. are important characteristics that affect subjective satisfaction for users, the curvature of the side display can give a different feeling to the user compared to the flat display [
10]. However, this issue has not been dealt with in previous studies. A few studies investigated the various facets of smartphones with curved screens. Ahn et al. [
11] investigated the effect of the curvature of the screen and hand size of the user on the physical comfort of using curved smartphones. Kwon et al. [
12], however, endeavored to determine the optimal curvature of the screen and thickness of the smartphone in terms of physical comfort using the muscular activities of the hand. However, they only focused on the curvature of the main screen. Yi et al. [
13] studied the effect of display curvature and hand length on the usability of smartphones. The results showed that the perceived grip comfort in two situations, calling and texting, was the highest when the hand length was medium. In addition, the value of the perceived grip comfort of the curved display was lower than that of the flat display for all hand lengths. However, this study did not compare perceived grip comfort between curvature displays. Therefore, it is necessary to study the grip comfort according to the specifications of the curvature display on the smartphone side.
Grip/usage pattern and personal information such as gender, age, and size of hand should be considered to determine the effect of the physical dimension of mobile phones on user satisfaction. Previous studies related to convenience for handheld devices were performed mainly on the assumption that only a single hand is used [
14]. This is because the size of most smartphones was small, and the use of a single hand was the main usage pattern. However, recently, as the size of smartphones has increased, it has become more common to use a smartphone with both hands. In addition, as the functions of smartphones become more diverse and the user interface gradually changes, various types of usage pattern have appeared. Lee et al. [
6] classified the type of the grip on the smartphone into three front touches and two rear touches. Le et al. [
15] classified the posture of using smartphones with one hand into three types: four-finger posture, small-finger posture, and clutch posture. Therefore, various usage patterns should be considered in determining the physical dimensions of smartphones.
In addition, as the screen size of smartphones is diverse, the demographic factors such as gender, age, and size of a user’s hand becomes one of the most important factors affecting the convenience of using a smartphone. Zhang and his colleagues [
16] confirmed that there are gender differences in metrics such as response time and hit rate for target selection on a smartphone with a large screen. Balakrishnan and Yeow [
17] figured out the influence of hand breadth, thumb length, and thumb circumference when typing characters on mobile phones. Lin [
18] analyzed the subjective fatigue according to the size of the smartphone by dividing the size of the human hand into three levels based on the hand length. Experimental results show that the small hand and medium hand have higher subjective fatigue as the size of the smartphone increases, and the opposite result is obtained in the case of large hands. These findings suggest that it is important to understand the relationship between demographic characteristics and the specifications of the smartphone when examining the subjective feel of smartphones.
Therefore, the purpose of this study is to understand the relationship between the curvature of the side display and the subjective feeling according to various usage patterns occurring in the smartphone and to compare the differences of subjective feeling. In addition, this study examines whether demographic variables such as gender and size of hand affect the usage patterns and design variables of smartphones. The remainder of this article is organized as follows.
Section 2 describes information about subjects and samples and the research methods.
Section 3 shows the results of the experiment. Finally, the discussions and conclusion of this study are given in
Section 4 and
Section 5, respectively.
4. Discussion
According to the ANOVA results, the samples with 4R and 6R were included in the third and second homogeneous subset for the front visibility, respectively. The control comfort for these two samples in use patterns (b) and (c) was included in the second homogeneous subset. In addition, the grip comfort for the sample with 10R in usage pattern (d) belonged to the second homogeneous subset. The sample with 8R was included in the first homogeneous subset for all analyses, while other samples were included in second or third homogeneous subset. There were some cases where other samples were evaluated better than the sample with 8R, however, there were no statistically significant differences in those cases. Thus, it could be concluded that 8R is the optimal radius for the curvature of the edge screen in a smartphone.
In the case of grip comfort, only a sample with 10R showed a statistically significant effect of the usage pattern on grip comfort. As a result of the post hoc analysis, it was confirmed that usage patterns (a), (d) and (c) had significant differences in 10R. In particular, the value of grip comfort in usage pattern (d) was found to be the lowest. Grip comfort is the one of the most common patterns for people using smartphones. Through this study, when the side curvature was 10R, it was confirmed that the value grip comfort for the usage pattern related to holding the smartphone was the lowest. Therefore, it can be seen that designing the side curvature as 10R should be avoided.
In the case of control comfort, it was confirmed that there was a statistically significant difference in samples and interaction effect between the sample and usage pattern. Through post hoc analysis, it was confirmed that there was a significant difference between samples in usage pattern (b) and (c). As a result, to maximize control comfort in these usage patterns, it is necessary to design a side curvature of 8R or more. In addition, a sample with 10R showed a statistically significant effect of the usage pattern on control comfort. At this time, it was found that statistical significance between the two groups of usage pattern ((a), (d) and (b), (c)) appeared. Therefore, since various types of usage patterns appear in smartphones, designing the side curvature with 10R seems to be a non-universal design.
In terms of stability, a repeated two-way ANOVA result showed no significant effect of curvature and usage pattern. According to Eardley et al. [
23], as the size of the smartphone gets bigger, security becomes lower. The samples used in this study had the same sized smartphone body, and there was a slight difference in size due to curvature. Therefore, as the size of the entire smartphone is almost the same, the curvature does not affect stability. In addition, Lee et al. [
24] found that there was no significant difference between the NASA-TLX metrics according to bezel size in the smartphone. According to Lee et al. [
24], the bezel was located at the bottom of the smartphone, and the curvature display was located on the right side in this study. Therefore, there is no difference in the subjective feeling of the user due to the size change of the smartphone by the bezel or the curvature display. However, the stability seems to be under the influence of grip comfort rather than control comfort because both stability and grip comfort showed similar results in terms of the evaluation of the curvature of the edge screen by usage pattern (
Table 12). The front visibility was significantly under the effect of the curvature of the edge screen; a larger curvature derived a higher front visibility. This seems reasonable for providing better visibility as the area of the edge screen increases. However, the difference in front visibility between 8R and 10R according to the post hoc results was insignificant; thus, 8R is adequate to provide good front visibility. In addition, as with front visibility, the value of side visibility was evaluated to be high as the curvature increased. However, the difference in the evaluation was not statistically significant. This seemed to be caused by the fixed thickness of the samples, which hindered the effect of the area increment in edge screens; when viewed edge-on, there were no differences in visible area by different curvatures.
Although in this study, there were no statistically significant differences identified in sample and usage patterns for demographic variables such as gender and hand size, it has been reported in previous studies that demographic variables have a significant effect on smartphone design [
17,
24,
25]. Balakrishnan and Paul [
25] showed that the subjective satisfaction increases as the hand size of male and female subjects decreases when people text on mobile phones. Lin [
18] conducted research that small-handed people experience increased subjective fatigue as the size of their smartphones increases, while those with larger hands show the opposite result. However, there was no significant difference in the results of independent t-tests between the small-hand and large-hand groups in the study. According to the research of Lee et al. [
6], there was no statistically significant difference between hand size and perceived discomfort.
Since most of the previous studies were conducted to understand the subjective feeling of the front display of the smartphone, it can be inferred from this study that there is a difference in the side display of the smartphone. Considering the reason for the research results, Lin [
18] performed a study to determine the degree of subjective comfort caused by the size of the smartphone relative to the size of the hand, with a difference of 0.5 inches between samples. In addition, the difference in the samples used in Xiong and Muraki [
26], who compared the area of thumb movement coverage in smartphones according to the size of the smartphone, was approximately 10 mm. From this point of view, the difference in sample width due to the curvature used in the study was less than 5 mm at maximum, because only the width of the bezel on the right side of the smartphone was the object of adjustment. Therefore, it is expected that the difference in width due to the curvature should be set to at least 10 mm in future studies.
5. Conclusions
Overall, this study investigated the effect of the curvature of the edge screen on users’ grip comfort, control comfort, stability, and visibility, based on subjective evaluation experiments. Especially, four representative usage patterns were defined and evaluated to investigate the interaction effect of curvature and usage patterns for grip comfort, control comfort, and stability. As a result, 8R was the most appropriate radius of curvature for edge screens when the overall results were considered. The results showed that the effect of curvature on grip and control comfort varies by usage pattern.
This study can be used as fundamental research to understand the subjective feeling in smartphones with a curvature display, which is being developed recently. However, this study has limitations based on surveys without using quantitative indices such as electromyography (EMG). In a future study, these quantitative indices to determine the relationship between subjective feelings and muscle fatigue due to changes in curvature of the screen can be conducted. In addition, it is expected that it will be possible to study the optimum curvature of display for various types of hands by considering not only the length of the hand but also the hand measurements such as the circumference and the finger.