## 3 Question-Answer-Response Dialogue Model
### Utterance Selection in Each State
This aspect should be important in dialogues for elderly people with declining cognitive ability. **Cons**. Participants who have no communication problems might have felt bored earlier during a dialogue if the dialogue was monotonous. * Choosing robot responses by using keyword match of user answers **Pros**. This method was clearly robust against speech recognition failures. In our question-answer-response dialogue model, if the speech recognition result contains words of the keyword attribute, the backchannel (comment) associated with the keyword is selected. Otherwise, the backchannel (comment) associated with no keyword (i.e., the default backchannel (comment)) is selected. Therefore, when the speech recognition result is a broken sentence, the default backchannel (comment) is selected in most cases. Because the default backchannel (comment) is a sentence that is coherent for any answer, the dialogue was usually coherent even if the speech recognition fails. **Cons**. There are two situations for a dialogue to break down. First, there is the case where a user asks a robot a question while the robot is in the \"answer mode\". Here, the sentence associated with the default attribute is selected unless it was time for the user to be posing a question to the robot. Because the sentence of the default attribute is not meaningful to the question, the dialogue would become unnatural. Second, there is the case where a keyword is matched owing to speech recognition failures, although this will rarely happen. For example, let us consider the following situation: a robot asks \"Which countries do you want to travel, France or England?\". Although a user answers France, the speech recognition result could be England. At this point, the dialogue would be strange because the robot would choose \"England\" as the response. To avoid this, we need more sophisticated algorithms. ### Application of the System

The ability to talk with elderly people is becoming increasingly important for social robots because dialogues play an important role in building human-robot relationships. Social robots instruct elderly people to take medicines, exercise, undergo cognitive training, and suggest lifestyle improvements in order to sustain physical and mental wellbeing. Instructions in such situations may not work well if the relationship with elderly people--in other words, a sense of trust, security, and familiarity--is not built. Conversely, if the relationship between elderly people and social robots is well formed, the instruction will be more effective. Therefore, not only robots as companions but also a various other robots will need to have a certain level of dialogue with humans. The proposed twin-robot dialogue system would be useful from the viewpoint that they can sustain dialogues up to 12 min 51 s.

Conceptualization, H.I. and Y.I. ; methodology, Y.Y. and T.I. ; software, M.C., T.A., and T.I. ; validation, Y.Y. and T.A. ; formal analysis, T.I. and M.C. ; investigation, T.I., Y.Y., and M.C. ; resources, Y.Y. and T.A. ; data curation, T.I. and M.C. ; writing--original draft preparation, T.I. ; writing--review and editing, T.I., Y.Y., and T.A. ; visualization, T.I. ; supervision, H.I. and Y.I. ; project administration, H.I. and Y.I. ; funding acquisition, H.I. and Y.I. All authors have read and agreed to the published version of the manuscript. This research was funded by NTT DOCOMO, INC., JST, PRESTO Grant Number JPMJPR1851, JST, ERATO Grant JPMJER1401 and JSPS KAKENHI Grant Number 19H05691. We appreciate participants and caregivers who had participated in the field trial, and the facility manager that had admitted us to do the field trial. [PERSON], [PERSON] and [PERSON] are working at NTT DOCOMO, INC., which is one of the funders of this study. ## References

* [PERSON] et al. (1988) [PERSON]; [PERSON]; [PERSON] Social relationships and health. _Science_**1988**, _241_, 540-545. [CrossRef] [PubMed]
* [PERSON] and [PERSON] (2009) [PERSON]; [PERSON] Social disconnectedness, perceived isolation, and health among older adults. _J.