## 2 Methods
### Experimental Design  
Testing was conducted with a one-factor within-subject design. The independent variable was the soft keyboard consisting of 26 alphabet letter keys on the keyboard section, as shown in Figure 1. In addition, the dependent variable was defined as whether to use the left or right thumb for the touch interaction with each letter key. The entire experiment consisted of three sessions: (1) preparatory; (2) main experimental; and (3) post-experimental sessions. Above all, in the preparatory session, two tasks were primarily conducted. First, the experimental postures of the participants were guided and controlled for testing. For instance, the participants were asked to take a seat in a chair and then had a chance to adequately set the height of the seat pan as preferred. They were also instructed to rest their elbows comfortably on a desk and to hold a given  
Figure 1: Experimental application. sartphone as naturally as possible in portrait mode with both hands as usual (Figure 2). However, in order to prevent the grip postures of both hands from being too unusual (e.g., too asymmetrical between the left and right hands), the participants were dictated to place their thumbs around the preset positions on the QWERTY keyboard section (i.e., around the letter keys S and D for the left thumb and around the letter keys J and K for the right thumb; Figure 1). Second, the participants had opportunities to directly manipulate the experimental application. No strict time limit was set, but it lasted approximately five minutes before the main experiments began. These practical trials were intended to reduce the learning effect by familiarizing the participants with the experimental application. During the main experimental session, data were collected on whether the participants employed their left or right thumb for touch interaction with each letter key. The testing was conducted in a small chamber in which temperature and illumination were maintained at approximately 21 \({}^{\circ}\)C and 300 lux to avoid interfering with the performance of the experimental tasks. The participants were asked not only to touch a given target key with their preferred thumb (no regional restriction) but also to do so as quickly and accurately as possible when the target letter appeared on the instruction section after a three-second countdown. Here, if the target key was missed, they needed to tap the key repetitively until the application conceded its successful touch. After the successful touch, the participants were also instructed to replace their thumbs with the preset positions before the next target letter key was displayed in the instruction section. The participants were asked to remain focused on tapping given target letter keys until all 26 alphabet letter keys appeared one by one. Note that the orders of the letter key appearance were completely randomized between participants and trials within participants because the application was programmed to display a randomly selected letter key at a time in a trial, as aforementioned. Each participant conducted 26 key tapping (i.e., 26 alphabet letters) in one experimental trial, and seven trials were repeated--a two-minute rest was given between the trials. Therefore, a total of 182 target letter keys (26 alphabet letters \(\times\) 7 repetitions) were processed by each participant. All the touch interactions during the experiments were video recorded under the consent of the participants, and the very first touch on each letter  
Figure 2: Experimental posture. key was only investigated to identify whether the participants employed their left or right thumb for touch interaction via video analysis. After all the experimental processes, a debriefing was conducted. A short interview was carried out with each participant to listen to their opinion on the experiments. The participants had opportunities to discuss the results of the experiments based on their daily basis experience with respect to the use of a smartphone QWERTY soft keyboard with two-thumb text entry.