Generating Indirect Answers to Yes-No Questions

An indirect answer to! a Yes-No question conversationally implicates the speaker's evaluation of the truth of the questioned proposition. We present the approach to generation used in our implemented system for generating and interpret~ing indirect answers to Yes-No questions in English. Generation of a discourse plan is performed in two phases: content planning and plan pruning. During content planning, stimulus conditions are used to trigger speaker goals to include appropriate extra information wit h the direct answer. Plan pruning determines what parts of this full response do not need to be stated explicitly resulting in, in appropriate discourse contexts, the generation of an indirect answer. 1. I n t r o d u c t i o n J Imagine a discourse context for (1) in which R's use i of just (ld) is intended to convey a No, i.e., that R is not going shopping tonight. (By convention, square brackets indicate that the enclosed text was not explicitly stated.) The part of R's response consisting of (ld) (le) is what we 'call an indirect answer to a YesNo question, and if (lc) had been uttered, (lc) would have been called a direct answer. l.a. O: I need a ride to the mall. b. Are you going shopping tonight? c. R: [no] d. My car's not running. e. The timing belt is broken. According to one study of spoken English [Ste84], 13 percent of responses to Yes-No questions were indirect answers. Thus, the ability to interpret indirect answers is required for robust dialogue systems. Furthermore, there are good reasons for generating indirect answers in a dialogue system. First, a direct yes or no alone may be misleading if extra information is needed to qualify the answer. Second, an indirect answer may contribute to a more efficient dialogue. For example, in addition to providing the requested information, it may anticipate a follow-up question from Q, or it may allow R to respond immediately without asking for clarification of the question. That is, the increased efficiency is more the result of avoiding the follow-up question or clarification subdialogue than the result of omitting the direct answer. Third, an indirect answer may be preferable for politeness considerations, as in (1). We have developed a computational model for the interpretation and generation of indirect answers to Yes-No questions in English. More precisely, by a YesNo question we mean one or more utterances used as a request by Q (the questioner) that R (the responder) convey R's evaluation of the truth of a proposition p. An indirect answer implicitly conveys via one or more utterances R's evaluation of the truth of the questioned proposition p, i.e. that p is true, that p is false, that there is some truth to p, that p may be true, or that p may be false. Our model presupposes that Q's question has been understood by P~ as intended by Q, that Q's request was appropriate, and that Q and I:t are engaged in a cooperative goal-directed dialogue. Both the interpretation and generation components have been implemented in Common Lisp on a Sun SPARCstation. A language user's pragmatic knowledge of how language is used to answer Yes-No questions in English can be used to constrain the problem of generating and interpreting indirect answers. This knowledge is encoded in our model as a set of domain-independent discourse plan operators and a set of coherence rules, described in section 2.1 Our system is reversible in that the same pragmatic knowledge is used by the generation and interpretation modules. Generation is modeled as discourse plan construction and interpretation as discourse plan inference. The output of generation is a discourse plan which can be realized by a tactical generation component [McK85], and the output of interpretation is the speaker's inferred discourse plan. We found that while the above pragmatic knowledge is sufficient for interpretation, it is not sufficient for the 1 Our main sources of data were previous studies [Hir85, Ste84], transcripts of naturally occurring two-person-dialogue lAme92], and constructed examples.