## 3 Methods
### Dialog System with Speech Level Control  
We describe a non-task-oriented dialog system that provides dynamic speech level control in dialog. A schematic of the dialog system is presented in Figure 1. The dialog system receives the text of the user's speech and performs morphological analysis using McCab ([[https://taku910.github.io/mecab/](https://taku910.github.io/mecab/)]([https://taku910.github.io/mecab/](https://taku910.github.io/mecab/)), accessed on 31 March 2025) [41]. The speech level of the user's utterance is identified based on the results of this analysis. This dialog system determines that the speech level is formal style when the user's utterance contains \"_desu_\" or \"_masu_\". Figure 1 shows an example case where the user's utterance is given in the formal style. The results of the morphological analysis are recorded in the user dialog log along with the utterance ID, utterance content, and formal style rate. The formal style rate (\(T_{formal}\)/\(T_{utterance}\)) is the proportion of utterances made in the formal style by the user during the dialog; it is calculated by counting the number of times the user used the formal style (\(T_{formal}\)) and dividing this by the total number of user utterances (\(T_{utterance}\)). The formal style rate is updated each time the user speaks. The speech level of the next utterance of the dialog system is determined based on the most recent formal style rate. Specifically, if the most recent formal style rate is greater than or equal to a threshold (\(\theta\)), the dialog system's utterance is set to be in the formal style; otherwise, it is set to be in the informal style. Based on the chosen speech level, the next specific utterance is chosen from the dialog system's utterance list. This system is designed to be rule-based (scenario-based), and the dialog progresses according to a single scenario that has been set in advance. We created a dialog scenario based on the dialog situation (see Table 2) that was adopted in the Situation Track of the Dialog System Live Competition 2 [42]. For each round of dialog, we prepared a list of dialog system utterances, with one formal-style and one informal-style candidate for each turn, and one candidate was selected based on the chosen speech level. The utterance that is finally selected by the dialog system is output to the user. The user then inputs a reply to the new utterance from the dialog system and the dialog proceeds in the same manner. The threshold value (\(\theta\)) was determined based on the formal style rate in the BTSJ Japanese natural conversation corpus [35]. Among the data in the corpus, we adopted the formal style rate of the first meeting dialog between university students who have the same relationship level as the threshold value. Specifically, the formal style rate was calculated based on the target conversation data using Equation (2). \[\text{Number of sentences containing ``_desu_'' or ``_masu_''/Number of all sentences.} \tag{2}\]  
The result was 29.6%, which was used as the threshold in this paper. This threshold was calculated based on data from natural conversations between humans and thus has a certain degree of reliability. In this study, the states in which users interacted with the dialog system were set as follows:  
* The user and dialog system are meeting for the first time and are of the same generation. * D and P are even. An example of dialog from the dialog system is depicted in Figure 2. The speech balloon on the left (white) represents a dialog system utterance, whereas that on the right (green) represents a user utterance. The dialog system says \"Nice to meet you\" as its first utterance, and the system uses a formal style. Thereafter, the speech level of the dialog system changes dynamically from a formal style to an informal style (bold part) according to the speech level (informal style) of the user.