## 2 Theoretical Background
### Psychological Approach
RG is mainly characterized by its ability to measure individual psychological evaluations through multiple statistical analysis methods quantitatively, and if the results from those are similar, confidence in conclusions can be increased [109]. Statistical analysis methods used in RG include PCA (principal components analysis), HCA (hierarchical cluster analysis), multidimensional scaling (MDS), and biplot. Among them, HCA, in particular, is mainly used in research such as psychotherapy because it is possible to easily analyze the elements that make up a single individual's psychology in the form of a tree. A representative method for non-grid-based methods is laddering techniques [111, 112]. Laddering is an in-depth interview technique based on means-end theory, where this approach is a method of observing and asking specific questions about product attributes and results [113]. Laddering involves a tailored interviewing format using a series of directed probes, with the express goal of determining sets of linkages between the critical perceptual elements across the range of attributes (A), consequences (C), and values (V) [114]. In other words, Laddering's process repeats to elicit configurations and derive new components by asking about the respondent's preferred structure questions such as \"Why is that important to you?\") [115]. We can use laddering to produce a direct and useful understanding of consumers while simultaneously seeing the connection of consumer perceptual processes [114]. Laddering is primarily used in marketing domains to understand consumer decision-making better but has also been successfully used as a research method for creating design recommendations in the HCI field. These researches used laddering and association techniques to understand why users choose a particular website over others [116], evaluate children's likeability of games [117], and develop a user-friendly mobile city applications [118]. The PCP-derived methodologies, RG and laddering, differ in their results and investigation methods, but they are essentially similar in their exploration. After all, both are methodologies that organize human psychological experiences and have characteristics whereby it is easy to identify perceptual elements for a target. An investigative methodology called the Zaltman metaphor elicitation technique (ZMET) is commonly used in marketing, which is similar in purpose to these two methodologies. Since people are not aware of their mental activities, and people's cognitive processes are complex on an unconscious level, it is not easy to recognize the cognitive process independently or express it in the language [119]. ZMET is a method for visualizing the cognitive processes of people, which allows consumers to collect their pictures to control the stimuli used in guided conversations. Through this, we can surface mental models that induce consumer thinking and behavior, and identify and characterize consumers' 'deep metaphor' [120]. ZMET is conducted as a guided conversation to allow participants in the experiment to collect their pictures. The guided conversation is a personal, one-on-one interview that includes a variety of steps, only a subset of which are used in any particular project [120]. Most of the studies using ZMET recruited and interviewed 10-20 participants [120; 121; 122; 123; 124] but there is also a study that experimented only with heavy users as needed [122]. In ZMET, 'deep metaphor' is a metaphor that structures what we think, act, and say. If deep metaphor is entirely unconscious,'surface metaphor' is the closest metaphor to daily life, and'metaphor theme' is a metaphor that exists under surface metaphor and is not entirely trapped in unconsciousness [125]. Deep metaphors and emotions work interchangeably, and without deep metaphors, it may be impossible to understand emotions [125; 126]. Many kinds of research using ZMET have included RG and laddering techniques in the interview process, as they are effective in deriving structures based on thought and behavior [120].