*3.1. States*

The four states in a dialogue are as follows: question, answer, backchannel, and comment. Figure 1 shows part of a dialogue constructed from the four states. The question state means the robot asks the user a question. The answer state means the user answers the robot. The backchannel state means the robot provides a backchannel, such as "uh-huh", "I see", or "really?". The comment state means that the robot comments on the question or the answer from the user.

**Figure 1.** Dialogue constructed from the four states.

There are three reasons for adopting such a structure:

• The structure makes a dialogue robust against speech recognition failures. Although speech recognition may fail, a robot can sustain a coherent dialogue by providing an ambiguous response; independent of user answer. Consider the dialogue in Figure 1. The user might answer "yes", "no", or "so-so" instead of "I don't remember". However, the response of the robot sounds coherent in any of the three ways. It means that the robot can sustain a coherent dialogue even when the speech recognition result is doubtful.

