## 2 Literature Review  
Application interface quality testing, called Usability Testing, is a technique used to assess the interface quality by a control group consisting of future potential users [10]. The purpose of testing is to check how users manage the system while performing typical (or project-defined) tasks. During these tests, researchers observe (including recording, using various techniques for further research) their behavior and interactions with the app's GUI. During the tests, users from the control groups perform tasks by implementing prepared software use scenarios. In this method, to identify problems related to usability, quantitative data related to participants' performance during the study, qualitative data (cognitive problems), and the level of satisfaction of research participants with the use of the application are examined. The study should involve homogeneous users (the so-called research group) who represent the indicated Personas. As experience shows, the size of a homogeneous research group (i.e., people with characteristics corresponding to those of the Persona in areas important for cooperation with the IT system) should be from 6 to 8 people [11]. Another criterion for selecting the research group is experience in using the IT system. When examining an interface, the main factor is the experience of using it. There are three types of users: novices (without any experience), experienced users and experts [12]. The interface requirements for each user group are different. Usability research uses various techniques to obtain data on the effectiveness and problems occurring when working with the IT system interface. These techniques are divided into objective and subjective [13]. Objective techniques allow measurement of work parameters such as the speed and quality of individual tasks, the number of clicks, the fact that a task was completed or not within the assumed time, the number of errors made or the number of calls for help. Subjective feelings about the quality of the interface are examined using sociological techniques, such as surveys or interviews with research participants. The literature contains many publications on the evaluation of user interfaces for mobile applications [14] and methods used during evaluation [15; 16], including the evaluation of educational applications [17; 18; 19]. Depending on the purpose of the assessment,the size of the system and the general conditions for conducting the study, researchers use different analysis methods. A combination of several different methods is also very often used [20; 21]. Prototyping methods [22; 23], heuristic analysis [24; 25], cognitive wandering [26; 27], expert tests [28], as well as a numerous methods with user participation are used, including scenario testing methods and survey methods, e.g., the SUS (System Usability Scale) survey [29]. More advanced techniques require specific tools. An example is eye tracking [30; 31]. This technique, as an objective method, allows for tracking the eye activity of subjects while using, e.g., a website, web or mobile application, and can be used for a multi-aspect analysis of GUI quality. This analysis may take into account not only the indication of weak and strong features and interface points, but also detailed user interaction with the application [32] and user activity in the application while performing specific tasks [33]. The combination of subjective methods, such as surveys and in-depth interviews, along with objective methods is very effective in the process of identifying errors in the GUI, even in a small research sample. This approach allows for identifying and hierarchizing problems related to the usability of the interface and indicating opportunities to remove errors [34; 35]. The aim of the paper is to present the quality testing stage of the T1 DCoach application interface using three research groups: children, their caregivers and educators.