## 5 Discussion
### Limitations and Future Work  
This study has several limitations. First, the experimental findings were limited to specific situations. We used Japanese dialog situations in which the speakers were the same age and meeting for the first time. However, experiments with situations in which the speakers are friends or of different ages are also necessary. Another limitation of the experiment is that it was conducted using only a text dialog system. Future experiments using interactive robots and virtual agents will provide useful knowledge for the research fields of human-agent and human-robot interaction. Similarly, interaction design to extend this work is expected to be realized for voice assistants that do not have physical characteristics [57]. Although speech level shifts are observed in a limited number of languages, many languages exhibit differences in the expression of respect, such as \"can\" and \"could\" in English; thus, it is possible to conduct research focusing on language forms in each country. Several studies have achieved the categorization and generation of polite utterances for the English language [58; 59]. Verification focusing on language and culture is considered to be  
Figure 4: Results of impression evaluation of low and high speech level recognition groups during text dialog experiment. The error bars represent the standard error.  
important in interaction research [60; 61; 62]. Additionally, this study focuses on changes in sentence endings in dialog and does not analyze the length of the dialog. In the future, it will be important to analyze not only the form of sentence endings (speech level) but also the quantity and quality of the dialog.  
Finally, the number and timing of speech level shifts in this experiment were not controlled because the speech level of the dialog system changed depending on the formal style rate of the user. However, as a speech level shift is generally performed in response to the dialog situation and the attitude of the interlocutor, it would be reasonable to design a dialog system that performs speech level shifts in response to the formal style rate of the user as an engineering study. Moreover, it may be possible to conduct experiments in which the number and timing of the speech level shifts are completely controlled as a psychological study.  
This experiment revealed that 41.3% of the participants performed speech level shifts in response to the speech level shifts performed by the dialog system; however, rigorously discussing the significance of this result is difficult (e.g., whether it exceeds the chance level), necessitating further research. This is because the research question posed by this study is relatively novel, and no existing data are available on the percentage of humans who engage in speech level shifts in response to dialog systems, complicating the determination of a general probability. Future research should consider methods for identifying the causes of speech level shifts performed by users, as well as a quantitative analysis of speech levels in human-dialog system dialog based on dialog data with sufficient quantity and quality.  
In this study, we used the simplified face violation estimation formula shown in Equation (1) and Table 1 to implement the dialog system; however, in real dialog, appropriate speech level shifting may not be possible without considering various demographic variables, including gender, age, and nationality. In this study, we conducted dialog experiments using the simplified personas listed in Table 2. Future research will be required to implement a complex politeness dialog model based on a wider range of variables.