DERECHOS Y OBLIGACIONES.

For a computer based chat system, the establishment of common ground with the user is a key success factor. Not only to make the conversation feel more human-like, but as an actual necessity for successful conversation with a human. This section will provide back- ground on grounding theory and discuss methods for grounding with VPINO.

2.4.1 Grounding Theory

The accumulation of mutual beliefs or common ground by the speakers holding a con- versation is a central part in conversation analysis [Cahn and Brennan, 1999, Paek and Horvitz, 1999, Traum, 1999, Brennan, 1998]. The term common ground was first described by Robert Stalnaker as “the common set of mutual knowledge that is taken for granted as the background of the conversation” [Stalnaker, 1978]. The accumulation of this knowledge is called grounding. The concept of grounding, the process of establishing mutual knowledge, beliefs and assumptions, was proposed by Clark and Brennan [Clark and Brennan, 1991]. However, common ground cannot be reached by one speaker alone. Both communication participants are needed to collaboratively establish this common ground.

The lack of a common ground in conversation has a huge impact: “When communi- cating partners fail to highlight the important points of their message to their partner or know the important points of the partner’s message, then both parties can never satisfy the their partner’s expectations (...) This lack of common ground damages interpersonal trust, especially when the communication partners do not share the contextual information of why the other party behaves the way they did.” [Cramton, 2001].

Reaching a common ground with a human user is a challenge for a conversational agent. Clark and Brennan identify eight characteristic constraints for communicating par- ties, namely copresence (collocation) , visibility, audibility, contemporaneity, simultaneity, se- quentiality, reviewability and revisibility. Text based chat communication largely differs from spoken conversation in human face-to-face coaching, e.g. by a lack of eye contact, mim- ics, gestures, and prosody. Also, conversation analysis presupposes a common ground between both communication partners prior to starting the conversation. For communica- tion between a computer system and a human user, such common knowledge or beliefs do not exist. User expectation is subjective and are mainly moderated by personal experience, either with human coaching or talking to computers in general. A lack of common ground leads to mutual misunderstanding, with the consequence of disappointment by both com- munication partners. The establishment and maintenance of common ground is essential for successful communication between VPINOand the client.

Clark and Brennan present a model that allows track the augmentation of common ground throughout the conversation [Clark and Brennan, 1991]. Their model distinguishes between the presentation phase and the acceptance phase. In the presentation phase, the speaker

presents an utterance. In the acceptance phase, the addressee shows acceptance of the pre- sented utterance by providing evidence of understanding. However, the display of accep- tance is not necessarily restricted to the use of language. For example, continued attention or nodding are also ways to show an utterance was received and understood.

The concept of presentation and acceptance are closely related to the perspective of “language as a sequence of action”: Presenting informations, showing understanding, agreeing or disagreeing on something can also be described as specific dialogue acts per- formed by a speaker and the receiver.

2.4.2 Methods for Grounding

This section presents a set of methods to reach common ground and evaluates their ap- plicability for a chat based conversational agent. As mentioned before, VPINOis a mixed

initiative system. Thus, in contrast to “stimulus-response” systems, VPINOshould be able to contribute to common ground in both presentation, and acceptance phase.

Contribution in the acceptance phase is a more complex task, especially for an artificial speaker such as VPINO. In the acceptance phase, the task is to display that a message has been received and understood, which is crucial to general communication. [Clark and Schaefer, 1989] describe five main methods to display understanding in the acceptance phase:

1. Continued Attention: The addressee is satisfied with the speaker’s presentation. Instead of interrupting the speaker, the addressee shows acceptance by continuing to attend.

2. Next Relevant Contribution: The addressee confirms grounding by starting in on a next relevant contribution.

3. Acknowledgement: The addressee utters an acknowledgement or a continuer such as ’yeah’, ’uh-huh’ or the like to express reception and understanding. 4. Display understanding: The addressee expresses understanding by displaying

verbatim of the speakers utterance, i.e. fully or partially repeating what the speaker just said.

5. Demonstrate understanding: The addressee demonstrates understanding by re- formulating, paraphrasing, collaborative completion of the speakers turn.

Continued attention is considered a weak method for grounding, Display and Demon- stration are considered the strongest methods to show understanding in conversation.

In communication settings without visual contact, such as online chat, continued at- tention is likely to provoke misunderstandings. When working with computer systems in

general, humans tend to interpret a lack of feedback as erroneous behaviour. As a result, continued attention is not a suitable method for grounding with a conversational agent. Keeping control of the conversation by next relevant contribution is a better way to establish common ground. However, timing the next contribution requires a suitable turn-taking mechanism (See Sect. 2.6).

Although the use of acknowledgement, continuer or back-channel as a grounding method is more frequent in spoken than in chat conversation, it is also crucial for a computer based system. The frequent utterance of acknowledgements is a simple, but effective way to cope with the problems of missing non-verbal feedback. A lack of acknowledgement is known to “(...) cause errors and confuse humans when talking to virtual agents” [Yankelovich et al., 1995]. The use of acknowledgement or continuer might seem an unimportant detail. Nevertheless, they have a great impact on successful human-like conversation.

Repetition of an utterance is considered a strong method of grounding in human con- versation. For a computer coach, repetition does not automatically prove that a message was also “understood”. Apart from rather “simple” acknowledgements as a display of understanding, humans often demand a proof of understanding. Demonstrating under- standing by paraphrasing or reformulating a speaker’s utterance requires detailed seman- tic knowledge about the utterance. Therefore, sub-dialogues include particular grounding phases for both presentation, and and acceptance phase, as will be presented in the follow- ing section.