## 2 Related Works  
### Usability Evaluation by Think-Aloud Protocol  
Think-aloud protocol methods are based on a theoretical framework initially developed by cognitive psychologists [PERSON] and [PERSON] [9] and were introduced to the field of usability testing by [PERSON] [2]. There are three types of think-aloud protocol methods [10; 11; 12]. The first type is the concurrent think-aloud protocol method in which the participants follow the think-aloud protocol at the same time as the experimental task is performed. The second type is the retrospective think-aloud protocol method in which the participants verbalize their thoughts after the experimental task is performed. The third type is a hybrid think-aloud protocol method in which the above two types are used together. For the concurrent think-aloud protocol method, participants speak in real time while operating the system, and it is the most commonly used protocol type for usability testing [6]. In addition to these three types of think-aloud protocol methods, new usability evaluation methods have also been proposed, such as a method that measures eye gaze in parallel with verbal responses and analyzes it together with the user's eye gaze information [13; 14]. [PERSON] and [PERSON] [15] pointed out that there was a difference between the original purpose of the think-aloud protocol and its use for usability testing. The original purpose of the think-aloud protocol was to study natural human cognitive processes. Therefore, participant interventions during the experiment should be avoided as much as possible since they could alter the participants' thought processes [10; 16]. However, usability testing aims to obtain the user's thoughts and also to discover usability problems. For this purpose, it is necessary to have the user speak as much as possible without silence. During usability testing, this is usually handled by asking the user to speak after a certain period of silence or by practicing speaking before starting the experiment. When intervening with the participants during the experiment, the experimenter's opinion should not be expressed. Instead, a neutral voice should be used. There are some challenges in using the think-aloud protocol for usability testing. One challenge is that the participants are required to speak during the operation, which is not a usual behavior. Some participants are not good at verbalizing their thoughts, and the complexity and duration of the task may affect the content and quantity of their verbal responses. The second challenge is that the analysis is time-consuming and requires a certain level of skill. In most cases, the think-aloud protocol is analyzed by reviewing the audio and video recordings of the experiment and transcribing the verbal data. According to a survey by [PERSON] et al. [7], many experts felt that this series of work was complicated. In addition, there was no straightforward procedure for extracting usability problems through analysis; therefore, experts had to devise their own way of organizing verbal data and observation notes, which required their skills and experience. In order to make this analysis more accessible and more efficient, some proposals have been made. For example, [PERSON] [17] analyzed what users verbalized during a think-aloud session and classified the verbalizations into four categories (reading, procedure, observation, and explanation). These categories could be used as templates for analysis. [PERSON] et al. [18] also studied a method to automatically detect usability problems from user's verbal responses and voice features by machine learning. Thus, when using think-aloudprotocol methods, it is crucial to make the analysis more accessible and the implementation more efficient. In this study, the aim was to make the analysis easier and more efficient by specifying a clear and formal analysis procedure for extracting usability problems using the think-aloud protocol.