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Discourse-based answering of why-questions
Employing RST structure
for finding answers to why
Suzan Verberne Lou Boves
Peter-Arno Coppen Nelleke Oostdijk
Department of Linguistics, Radboud University Nijmegens.verberne|l.boves|p.a.coppen|n.oostdijk@let.ru.nl ABSTRACT. This paper presents the work that we have carried out in investigating the purpose of discourse structure for why-question answering (why-QA). We developed a system for answer-ing why-questions that employs the discourse relations in a pre-annotated document collection(the RST Treebank). With this method, we obtain a recall of 53.3% with a mean reciprocal rank(MRR) of 0.662. We argue that the maximum recall that can be obtained from the use of RSTrelations as proposed in the present paper is 58.0%. If we discard the questions that requireworld knowledge, maximum recall is 73.9%. We conclude that discourse structure can play animportant role in complex question answering, but that more forms of linguistic processing areneeded for increasing recall. RÉSUMÉ. Cet article présente la recherche que nous avons réalisée en examinant le but de la structure du discours de réponse aux questions du type pourquoi (why-QA). Nous avons dé-veloppé un système de réponse aux questions pourquoi qui utilise les relations RST dans unecollection de documents pré-annotés (le RST Treebank). En appliquant cette méthode, nousobtenons un rappel de 53,3 % avec une Moyenne du Rang Inverse (MRR) de 0,662. Nous soute-nons que le rappel maximum qui puisse être obtenu en utilisant les relations RST est de 58,0 %.
En supprimant les questions qui requièrent une connaissance du monde, le rappel maximalserait de 73,9 %. Nous concluons que les structures du discours peuvent jouer un rôle impor-tant dans la réponse aux questions complexes, mais que l’augmentation du rappel nécessitedavantage de sortes de traitements linguistiques.
KEYWORDS: Question answering, why-questions, discourse structure, RST MOTS-CLÉS : Répondre questions, questions pourquoi, structure du discours, RST TAL Volume 47 – n˚2/2007, pages 21 à 41 1. Introduction
Up to now, why-questions have largely been ignored by researchers in the field of question answering (QA). One reason for this is that the frequency of why-questionsposed to QA systems is lower than that of other types of questions such as who- andwhat-questions (Hovy et al., 2002). However, why-questions cannot be neglected:as input for a QA system, they comprise about 5 percent of all wh-questions (Hovyet al., 2001; Jijkoun and De Rijke, 2005) and they do have relevance in QA applica-tions (Maybury, 2003). A second reason why this type of question has largely beendisregarded until now is that the techniques that have proven to be successful in QAfor closed-class questions have been demonstrated to be not suitable for questions thatexpect an explanatory answer instead of a noun phrase (Kupiec, 1999).
Researchers in the field of discourse analysis have investigated whether knowl- edge about discourse structure can be put to use in a number of applications, amongwhich language generation, text summarization, and machine translation (Carlsonet al., 2003). The relevance of discourse analysis for QA applications has been sug-gested by Marcu and Echihabi (2001) and Litkowski (2002). Breck et al. (2000)suggest that knowledge about discourse relations would have allowed their system forTREC-8 to answer why-questions. In this paper we take on the challenge and investi-gate to what extent discourse structure does indeed enable answering why-questions.
In the context of our research, a why-question is defined as an interrogative sen- tence in which the interrogative adverb why (or a synonymous word or phrase) occursin (near) initial position. Furthermore, we only consider the subset of why-questionsthat could be posed to a QA system (as opposed to questions in a dialogue or in alist of frequently asked questions) and for which the answer is known to be presentin some related document collection. In particular, our research is limited to ques-tions obtained from a number of subjects who were asked to read documents from thecollection and formulate why-questions that another person would be able to answergiven the text.
The answer to a why-question is a clause or sentence (or a small number of coher- ent sentences) that answers the question without adding supplementary and redundantcontext. The answer is not necessarily literally present in the source document, but itmust be possible to deduce it from the document.
An approach for automatically answering why-questions, like general approaches for factoid-QA, will involve at least four subtasks: (1) question analysis and querycreation, (2) retrieval of candidate paragraphs or documents, (3) analysis and selec-tion of text fragments, and (4) answer generation. In the current research, we wantto investigate whether structural analysis and linguistic information can make QA forwhy-questions feasible. In previous work (Verberne, 2006), we focused on questionanalysis for why-questions. From other research reported on in the literature it ap-pears that knowing the answer type helps a QA system in selecting potential answers.
Therefore, we created a syntax-based method for the analysis of why-questions thatwas aimed at predicting the semantic answer type. We defined the following answer types for why-questions, based on Quirk et al. (1985): motivation, cause, circum-stance and purpose. Of these, cause (52%) and motivation (37%) are by far the mostfrequent types in our set of why-questions pertaining to newspaper texts. With oursyntax-based method, we were able to predict the correct answer type for 77.5% ofthese questions (Verberne et al., 2006b).
After analysis of the input question, the QA system will retrieve a small set of documents that possibly contain the answer. Analysis of the retrieved documents isthen needed for extracting potential answers. Thus, a system for why-QA needs atext analysis module that yields a set of potential answers to a given why-question.
Although we now have a proper answer type determination approach, the problemof answer extraction is still difficult. As opposed to factoid-QA, where named entityrecognition can play an important role in the extraction of potential answers, findingpotential answers to why-questions is still an unsolved problem. This means that weneed to investigate how we can recognize the parts of a text that are potential answersto why-questions.
We decided to approach this answer extraction problem as a discourse analysis task. In this paper, we aim to find out to what extent discourse analysis can help inselecting answers to why-questions. We also investigate the possibilities of a methodbased on textual cues, and used that approach as baseline for evaluating our discourse-based method. Below, we will first introduce RST as a model for discourse analysis.
Then we present our method for employing RST for why-QA, followed by the resultsthat we obtained. We conclude this paper with a discussion of the limitations andpossibilities of discourse analysis for the purpose of why-QA and the implications forfuture work.
2. Rhetorical Structure Theory (RST)
The main reasons for using RST as a model for discourse structure in the present research are the following. First, a treebank of manually annotated English texts withRST structures is available for training and testing purposes. This RST DiscourseTreebank, created by (Carlson et al., 2003), contains a selection of 385 Wall StreetJournal articles from the Penn Treebank that have been annotated with discourse struc-ture in the framework of RST. Carlson et al. adapted the default set of discourse rela-tions proposed by Mann and Thompson for the annotation of the Wall Street Journalarticles in the treebank. The annotations by Carlson et al. are largely syntax-based,which fits the linguistic perspective of the current research. A second reason for usingRST is that relatively good levels of agreement have been measured between humanannotators of RST, which indicates that RST analyses do not strongly depend on sub-jective interpretations of the structure of a text (Bosma, 2005).
In RST, the smallest units of discourse are called elementary discourse units (EDUs). In terms of the RST model, a rhetorical relation typically holds betweentwo EDUs, one of which (the nucleus) is more essential for the writer’s intention than the other (the satellite). If two related EDUs are of equal importance, there is a mult-inuclear relation between them. Two or more related EDUs can be grouped togetherin a larger span, which in its turn can participate in another relation. By grouping andrelating spans of text, a hierarchical structure of the text is created. In the remainderof this paper, we will refer to such a hierarchical structure as an RST tree.
3. Our method for discourse-based why-QA
3.1. Main ideas and procedure
Let us consider a why-question-answer pair and the RST structure of the corre- sponding source text. We hypothesize the following: 1. The question topic1 corresponds to a span of text in the source document and the answer corresponds to another span of text; 2. In the RST structure of the source text, an RST relation holds between the text span representing the question topic and the text span representing the answer.
If both hypotheses are true, then RST can play an important role in answering why-questions.
For the purpose of testing our hypotheses, we need a number of RST annotated texts and a set of question-answer pairs that are linked to these texts. Therefore, weset up an elicitation experiment using the RST Treebank as data set. We selected seventexts from the RST Treebank of 350–550 words each. Then we asked native speakersto read one of these texts and to formulate why-questions for which the answer couldbe found in the text. The subjects were also asked to formulate answers to each of theirquestions. This resulted in a set of 372 why-question and answer pairs, connectedto seven texts from the RST Treebank. On average, 53 question-answer pairs wereformulated per source text. There is much overlap in the topics of the questions, as wewill see later.
A risk of gathering questions following this method, is that the participants may feel forced to come up with a number of why-questions. This may lead to a set ofquestions that is not completely representative for a user’s real information need. Webelieve however that our elicitation method is the only way in which we can collectquestions connected to a specific (closed) set of documents. We will come back to therepresentativeness of our data collection in section 5.3.
We performed a manual analysis on 336 of the collected question-answer pairs in order to check our hypotheses – we left out the other (randomly selected) pairs forfuture testing purposes (not addressed in the current paper). We chose an approach 1. The topic of a why-question is the proposition that is questioned. A why-question has theform ‘WHY P?’, in which the proposition P is the topic. (Van Fraassen, 1988) in which we analyzed our data according to a clear step-by-step procedure, which weexpect to be suitable for answer extraction performed by a QA system. This means thatour manual analysis will give us an indication of the upper bound of the performancethat can be achieved using RST following the proposed approach.
First, we selected a number of relation types from Carlson et al.’s relation set, which we believed might be relevant for why-QA. We started with the four answertypes mentioned in the introduction of this paper (cause, purpose, motivation andcircumstance), but it soon appeared that there is no one-to-one relation between thefour classes we defined based on Quirk et al. (1985) and relation types in Carlsonet al.’s set. For instance, Carlson et al.’s relation set does not contain the relation typemotivation, but uses reason instead. Moreover, we found that the set of relations towhich at least one why-question in our data collection refers is broader than just cause,circumstance, purpose and reason. Therefore, we extended the list during the manualanalysis. The final set of selected relations is shown in Table 1.
Table 1. Selected relation types
For the majority of these relations, the span of text that needs explanation (or elaboration, evidence, etc.) is the nucleus of the relation, and the span of text givingthis explanation is the satellite. The only exception to this rule is the cause relation,where the cause is given by the nucleus and its result by the satellite. Knowing this,we used the following procedure for analyzing the questions and answers: I. Identify the topic of the question.
II. In the RST tree of the source document, identify the span(s) of text that ex- press(es) the same proposition as the question topic.
III. Is the found span the nucleus of a relation of one of the types listed in Table 1 (or, in case of cause relations, the satellite)? If it is, go to IV. If it is not, go to V.
IV. Select the related satellite (or nucleus in case of a cause relation) of the found The effects of the procedure can best be demonstrated by means of an example.
Consider the following question, formulated by one of the native speakers after he hadread a text about the launch of a new TV channel by Whittle Communications L.P.
Q: Why does Christopher Whittle think that Channel One will have no difficulties The topic of this question is Christopher Whittle thinks that Channel One will haveno difficulties in reaching its target. According to our first hypothesis, the propositionexpressed by the question topic matches a span in the RST structure of the sourcedocument. We manually selected the following text fragment which expresses theproposition of the question topic: “What we’ve done in eight weeks shows we won’t have enormous diffi-culties getting to the place we want to be”, said Mr. Whittle.
This sentence covers span 18–22 in the corresponding RST tree, which is shown Figure 1. RST sub-tree for the text span “What we’ve done in eight weeks shows we
won’t have enormous difficulties getting to the place we want to be, said Mr. Whittle.”

In this way, we tried to identify a span of text corresponding to the question topic In cases where we succeeded in selecting a span of text in the RST tree corre- sponding to the question topic, we searched for potential answers following step IIIand IV from the analysis procedure. As we can see in Figure 1, the span What we’vedone in eight weeks shows we won’t have enormous difficulties getting to the place wewant to be, said Mr. Whittle is the nucleus of an evidence relation. Since we assumedthat an evidence relation may lead to a potential answer (Table 1), we can select thesatellite of this relation, span 23–28, as an answer (see Figure 2 below): A: He said his sales force is signing up schools at the rate of 25 a day. In California and New York, state officials have opposed Channel One. Mr. Whittle saidprivate and parochial schools in both states will be canvassed to see if they areinterested in getting the programs.
Figure 2. RST sub-tree containing the satellite span “He said his sales force . to see
if they are interested in getting the programs.”

We analyzed all 336 why-questions following this procedure. The result of this manual analysis is a table containing all questions and for each question the followingfields: (a) the manually identified topic from the source text with its correspondingspan from the RST tree; (b) the answer span that we found for the question topic; (c)the type of relation that holds between topic span and answer span, if there is a relation;and (d) information about whether the answer found is correct. We will come back tothis in section 4.1, where we discuss the outcome of the manual analysis.
3.2. Implementation
We implemented the procedure presented above in a Perl script. In section 3.1, we assumed that the RST structure can lead to a possible answer span once the topic spanhas been identified as a nucleus of a relevant relation. Therefore, the most critical taskof our procedure is step II: to identify the span(s) of text that express(es) the sameproposition as the question topic.
Since we are only interested in those spans of text that participate in an RST re- lation (step III), we need a list of all nuclei and satellites for each document in ourdata collection, so that our system can select the most relevant nuclei. Therefore,we built an indexing script that takes as input file the RST structure of a document, and searches it for instances of relevant relations (Table 1). It then extracts for eachrelation its nucleus, satellite and relation type and saves it to an index file (in plaintext). In case of a multinuclear relation, the script saves both nuclei to the index file.
Moreover, cause relations are treated a bit differently from the other relation types. Incause relations, as explained before, the span of text describing the cause is markedas nucleus, not as satellite. Thus, the satellite of cause relations should be indexed formatching to the question topic instead of the nucleus. Therefore, nucleus and satelliteare transposed when indexing cause relations. Below, where we use the term nucleusin describing the retrieval process, we mean the satellite for cause relations and thenucleus for all other relations.
Figures 3 and 4 below illustrate the conversion from an RST structure file to an index file. We created indexes for all documents in the RST Treebank.
Figure 3. Fragment of the original RST structure
Figure 4. Fragment of the resulting index
For the actual retrieval task, we wrote a second Perl script that takes as input one of the document indices, and a question related to the document. Then it performs thefollowing steps: 1. Read the index file and normalize each nucleus in the index. Normalization includes at least removing all punctuation from the nucleus. Other forms ofnormalizing that we explored are lemmatization, applying a stop list, and addingsynonyms for each content word in the nucleus. These normalization forms arecombined into a number of configurations, which are discussed in section 4.2; 2. Read the question and normalize it, following the same normalization procedure 3. For each nucleus in the index, calculate the likelihood P (Nucleus | Question) using the following language model (N = nucleus; Q = question; R = relationtype for nucleus): Nucleus likelihood P (N |Q) ∼ P (Q|N ) · P (N ) Question likelihood P (Q|N ) = # question words in nucleus Relation Prior P (R) = # instances of this relation type in question set # occurrences of this relation type in treebank For calculation of the relation prior P(R), we counted the number of occurrencesof each relation type in the complete RST Treebank. We also counted the num-ber of occurrences to which at least one question in our data collection refers.
The proportion between these numbers, the relation prior, is an indication of therelevance of the relation type for why-question and answer pairs.
For convenience, we take the logarithm of the likelihood. This avoids underflowproblems with very small probabilities. Thus, since the range of the likelihoodis [0.1], the range of the logarithm of the likelihood is [-∞.0]; 4. Save all nuclei with a likelihood greater than the predefined threshold (see sec- 5. Rank the nuclei according to their likelihood; 6. For each of the nuclei saved, print the corresponding answer satellite and the We measured the performance of our implementation by comparing its output to theoutput of the manual analysis described in section 3.1.
4. Results
In this section, we will first present the outcome of the manual analysis, which gives an indication of the performance that can be achieved by a discourse-based sys-tem for why-QA (section 4.1).
Then we present the performance of the current version of our system. When presenting the results of our system, we can distinguish two types of measurements.
First, we can measure the system’s absolute quality in terms of recall and mean recip-rocal rank (MRR). Second, we can measure its performance relative to the results weobtained from the manual analysis. In this section, we do both (section 4.2).
4.1. Results of the manual analysis
As described in section 3.1, our manual analysis procedure consists of four steps: (I) identification of the question topic, (II) matching the question topic to a span oftext, (III) checking whether this span is the nucleus of an RST relation (or satellite,in case of a cause relation), and (IV) selecting its satellite as answer. Below, we willdiscuss the outcome of each of these sub-tasks.
The first step succeeds for all questions, since each why-question has a topic. For the second step, we were able to identify a text span in the source document thatrepresents the question topic for 279 of the 336 questions that we analyzed (83.0%).
We found that not every question corresponds to a unique text span in the sourcedocument. For these 279 questions, we identified 84 different text spans. This meansthat on average, each text span that represents at least one question topic is referredto by 3.3 questions. For the other 57 questions, we were not able to identify a textspan in the source document that represents the topic. These question topics are notexplicitly mentioned in the text but inferred by the reader using world knowledge. Wewill come back to this in section 5.1.
For 207 of the 279 questions that have a topic in the text (61.6% of all questions), the question topic participates in a relation of one of the types in Table 1 (step III).
Evaluation of the fourth step, answer selection, needs some more explanation. For each question, we selected as an answer the satellite that is connected to the nucleuscorresponding to the question topic. For the purpose of evaluating the answers foundusing this procedure, we compared them to the user-formulated answers. If the answerfound matches at least one of the answers formulated by native speakers in meaning(not necessarily in form), then we judged the answer found as correct. For exam-ple, for the question Why did researchers analyze the changes in concentration of twoforms of oxygen?, two native speakers gave as an answer To compare temperaturesover the last 10,000 years, which is exactly the answer that we found following ourprocedure. Therefore, we judged our answer as correct, even though eight subjectsgave a different answer to this question. Evaluating the answer that we found to thequestion Why does Christopher Whittle think that Channel One will have no difficul-ties in reaching its target? is slightly more difficult, since it is longer than any ofthe answers formulated by the native speakers. We got the following user-formulatedanswers for this question: (1) Because schools are subscribing at the rate of 25 a day.
(2) Because agents are currently signing up 25 schools per day.
(3) He thinks he will succeed because of what he has been able to do so far.
(4) Because of the success of the previous 8 weeks.
Answers 1 and 2 refer to leaf 24 in the RST tree (see Figure 2); answers 3 and 4 referto leaf 18 in the tree (see Figure 1). None of these answers correspond exactly to the span that we found as answer using the answer extraction procedure (He said hissales force . in getting the programs.). However, since some of the user-formulatedanswers are part of the answer span found, and because the answer is still relativelyshort, we judged the answer found as correct.
We found that for 195 questions, the satellite connected to the nucleus correspond- ing to the topic is a correct answer. This is 58.0% of all questions.
The above figures are summarized in Table 2.
Table 2. Outcome of manual analysis
Questions for which we identified a text span corresponding to the topicQuestions for which the topic corresponds to the nucleus of a relation (or satellite in case ofa cause relation)Questions for which the satellite of this rela- In section 5.1, we will come back to the set of questions (42%) for which our 4.2. System evaluation
We evaluate our system using the outcome of our manual analysis as reference.
We used the answer that we found during manual analysis as reference answer. Wemeasured recall (the proportion of questions for which the system gives at least thereference answer) and MRR (1/rank of the reference answer, averaged over all ques-tions.) We also measured recall as proportion of the percentage of questions for whichthe manual analysis led to the correct answer (58%, see Table 2 above).
We tested a number of configurations of our system, in which we varied the fol- 1. Applying a stop list to the indexed nuclei, i.e. removing occurrences of 251 high-frequent words, mainly function words; 2. Applying lemmatization, i.e. replacing each word by its lemma if it is in the CELEX lemma lexicon (Baayen et al., 1993). If it is not, the word itself is kept; 3. Expanding the indexed nuclei with synonym information from WordNet (Fellbaum, 1998), i.e. for each content word in the nucleus (nouns, verbs andadjectives), searching the word in WordNet and adding to the index all lemmasfrom its synonym set; 4. Changing weights between stop words and non-stop words.
We found that best performing is the configuration in which stop words are not re-moved, lemmatization is applied, no synonyms are added, and stop words and non-stop words are weighted 0.1/1.9. Moreover, in order to reduce the number of answersper question, we added a threshold to the probability of the nuclei found. For decidingon this threshold, we investigated what the log probability is that our system calcu-lates for each of the correct (reference) answers in our data collection. As threshold,we chose a probability that is slightly lower than the probabilities of these referenceanswers.
For measuring the performance of our system, we added a function to the system in Perl that compares the answer spans found by the system to the answer in thereference table that was manually created (see section 3.1). We ran our system on the336 questions from our data collection.
With the optimal configuration as described above, the system found the reference answer for 179 questions. So, the system obtains a recall of 53.3% (179/336). Thisis 91.8% of the questions for which the RST structure led to the correct answer in themanual analysis (179/195). The average number of answers that the system gives perquestion is 16.7. The mean reciprocal rank for the reference answer is fairly high:0.662. For 29.5% of all questions, the reference answer is ranked in first position.
This is 55.3% of the questions for which the system retrieved the reference answer.
An overview of the system results is given in Tables 3 and 4 below.
We should note here that recall will go up if we add synonyms to the index for all nuclei, but this lowers MRR and heavily slows down the question-nucleus matchingprocess.
Table 3. Main results for optimal configuration
Recall as proportion of questions for which the RST structure can lead to a correct answer (%)Average number of answers per question Table 4. Ranking of reference answer
Reference answer ranked in 2nd to 10th position Reference answer ranked in other position 5. Discussion of the results
In the discussion of the results that we obtained, we will focus on two groups of questions. First, we will discuss the questions for which we could not find an answerin our manual analysis following the procedure proposed (procedure shortcomings).
Second, we will consider the questions for which we found an answer using manualanalysis but our system could not find this answer (system shortcomings). For bothgroups of questions, we will study the cases for which we did not succeed, and makerecommendations for future improvements of our system. In the last part of this sec-tion, we will give an overview of the types of RST relations that were found to play arole in why-QA.
5.1. Discussion of procedure shortcomings
We reported in section 4.1 that for 195 why-questions (58.0% of all questions), the answer could be found after manually matching the question topic to the nucleus of anRST relation and selecting its satellite as answer. This means that for 141 questions(42.0%), our method did not succeed. We distinguish four categories of questions forwhich we could not extract a correct answer using this method (percentages are givenas part of the total of 336 questions): 1. Questions whose topics are not or only implicitly supported by the source text (57 questions, 17.0%). Half of these topics is supported by the text, but onlyimplicitly. The propositions underlying these topics are true according to thetext, but we cannot denote a place in the text where this is confirmed explicitly.
Therefore, we were not able to select a span corresponding to the topic. Forexample, the question Why is cyclosporine dangerous? refers to a source textthat reads They are also encouraged by the relatively mild side effects of FK-506,compared with cyclosporine, which can cause renal failure, morbidity, nauseaand other problems. We can deduce from this text fragment that cyclosporineis dangerous, but we need knowledge of the world (renal failure, morbidity,nausea and other problems are dangerous) to do this. For the other half of thesequestions, the topic is not supported at all by the text, even not implicitly. Forexample, Why is the initiative likely to be a success?, whereas nowhere in thetext there is evidence that the initiative is likely to be a success.
2. Questions for which both topic and answer are supported by the source text but there is no RST relation between the span representing the question topic spanand the answer span (55 questions, 16.4%). In some cases, this is because thetopic and the answer refer to the same EDU. For example, the question Whywere firefighters hindered? refers to the span Broken water lines and gas leakshindered firefighters’ efforts, which contains both question topic and answer. In other cases, question topic and answer are embedded in different, non-relatedspans, which are often remote from each other.
3. Questions for which the correct (i.e. user-formulated) answer is not or only im- plicitly supported by the text (17 questions, 5.1%). In these cases, the questiontopic is supported by the text, but we could not find evidence in the text thatthe answer is true or we are not able to identify the location in the text where itis confirmed explicitly. For example, the topic of the question Why was GerryHogan interviewed? corresponds to the text span In an interview, Mr. Hogansaid. The native speaker that formulated this question gave as answer Becausehe is closer to the activity of the relevant unit than the Chair, Ted Turner, sincehe has the operational role as President. The source text does read that Mr.
Hogan is president and that Ted Turner is chair, but the assumption that GerryHogan is closer to the activity than Ted Turner has been made by the reader, notby the text.
4. Questions for which the topic can be identified in the text and matched to the nucleus of a relevant RST relation, but the corresponding satellite is not suitableor incomplete as answer (12 questions, 3.6%). These are the questions thatin table 2 make the difference between the last two rows (207-195). Someanswers are unsuitable because they are too long. For instance, there are caseswhere the complete text is an elaboration of the sentence that corresponds tothe question topic. In other cases, the answer satellite is incomplete comparedto the user-formulated answers. For example, the topic of the question Why didHarold Smith chain his Sagos to iron stakes? corresponds to the nucleus of acircumstance relation that has the satellite After three Sagos were stolen fromhis home in Garden Grove. Although this satellite gives a possible answer tothe question, it is incomplete according to the user-formulated answers, whichall mention the goal To protect his trees from thieves.
Questions of category 1 above cannot be answered by a QA system that expects the topic of an input question to be present and identifiable in a closed document col-lection. If we are not able to identify the question topic in the text manually, then aretrieval system cannot either. A comparable problem holds for questions of category3, where the topic is supported by the source text but the answer is not or only im-plicitly. If the system searches for an answer that cannot be identified in a text, thesystem will clearly not find it in that text. In the cases where the answer is implic-itly supported by the source text, world knowledge is often needed for deducing theanswer from the text, like in the examples of cyclosporine and Gerry Hogan above.
Therefore, we consider the questions of types 1 and 3 as unsolvable by a QA systemthat searches for the question topic in a closed document collection. Together thesecategories cover 22.0% of all why-questions.
Questions of category 2 (16.4% of all questions) are the cases where both ques- tion topic and answer can be identified in the text, but where there is no RST relationbetween the span representing the question topic span and the answer span. We can search for ways to extent our algorithm so that it can handle some of the cases men-tioned. For instance, we can add functionality for managing question-answer relationson sub-EDU level. We think that in some of these cases, syntactic analysis can helpin extracting the relation from the EDU. The example question above, Why were fire-fighters hindered? can be answered by a QA system if it knows that the question canbe rephrased by What hindered firefighters?, and that has syntactic information aboutthe EDU Broken water lines and gas leaks hindered firefighters’ efforts. The risk ofadding functionality for cases like this is that the number of possible answers per ques-tion will increase, decreasing the MRR. We should investigate to what extent syntacticanalysis can help in cases where the answer lies in the same EDU as the question. Forcases where question topic and answer are embedded in non-related spans, we canat the moment not propose smart solutions that will increase recall without heavilydecreasing the MRR. The same holds for questions of category 4 (3.3%), where RSTleads to an answer that is incomplete or unsuitable.
We can conclude from this analysis that there is a subset of why-questions (22.0%) that cannot be answered by a QA system that uses a closed document collection sinceknowledge of the world is essential for answering these questions. Moreover, there is afurther subset of why-questions (16.4% + 3.6%) that cannot be answered by a systemthat uses RST structure only, following the approach that we proposed. Together,this means that 42.0% of why-questions cannot be answered following the suggestedapproach. Thus, the maximum recall that can be achieved with this method is 58.0%.
If we discard the 72 (57+15) questions that require world knowledge, maximum recallwould be 73.9% (195/(336-72)).
In order to judge the merits of RST structure for why-QA, we investigated the possibilities of a method based on textual cues (without discourse structure). To thatgoal, we analyzed the text fragments related to each question-answer pair in our datacollection. For each of these pairs, we identified the item in the text that indicates theanswer. For 50% of the questions, we could identify a word or group of words that inthe given context is a cue for the answer. Most of these cues, however, are very fre-quent words that also occur in many non-cue contexts. For example, the subordinatorthat occurs 33 times in our document collection, only 3 of which are referred to by oneor more why-questions. This means that only in 9% of the cases, the subordinator thatis a why-cue. The only two words for which more than 50% of the occurrences arewhy-cues, are because (for 4.5% of questions) and since (2.2%). Both are a why-cuein 100% of their occurrences. For almost half of the question-answer pairs that donot have an explicit cue in the source text, the answer is represented by the sentencethat follows (17.6% of questions) or precedes (2.8%) the sentence that represents thequestion.
Having this knowledge on the frequency of cues for why-questions, we defined the I. Identify the topic of the question.
II. In the source document, identify the clause(s) that express(es) the same propo- III. Does the clause following the matched clause start with because or since? If it does, go to IV. If it does not, go to V.
IV. Select the clause following the matched clause as answer.
V. Select the sentence following the sentence containing the matched clause as A system that follows this baseline method can obtain a maximum recall of 24.3% (4.5+2.2+17.6). This means that an RST-based method can improve recall by almost140% compared to a simple cue-based method (58.0% compared to 24.3%).
5.2. Discussion of system shortcomings
There are 22 questions for which the manual analysis led to a correct answer, but the system did not retrieve this reference answer. For 17 of them, the nucleus that wasmatched to the question topic manually, is not retrieved by the system because thereis no (or too little, given the threshold) lexical overlap between the question and thenucleus that represents its topic. For example, the question Why are people stealingcycads? can be matched manually to the span palm-tree rustling is sprouting up allover Southern California, but there are no overlapping words. If we add synonymsto our index for each nucleus (see section 3.2), then 10 of these questions can beanswered by the system, increasing recall.
For three other questions, it is our algorithm that fails: these are cases where the question topic corresponds to the satellite of an elaboration relation, and the answerto the nucleus, instead of vice versa. We implemented this functionality for causerelations (see section 4.2), but implementing it for elaboration relations, where thesetopic-satellite correspondences are very rare, would increase the number of answersper questions and decrease MRR without increasing recall very much.
5.3. RST relations that play a role in why-QA
We counted the number of occurrences of the relation types from Table 1 for the 195 questions where the RST relation led to a correct answer. This distribution ispresented in Table 5. The meaning of the column Relative frequency in this contextwill be explained below.
As shown in table 5, the relation type with most referring question-answer pairs, is the very general elaboration relation. It seems striking that elaboration is more Table 5. Addressed relation types
frequent as a relation between a why-question and its answer than reason or cause.
However, if we look at the relative frequency of the addressed relation types, we seeanother pattern: in our collection of seven source texts, elaboration is a very frequentrelation type. In the seven texts that we consider, there are 143 occurrences of anelaboration relation. Of the 143 nuclei of these occurrences, 16 were addressed byone or more why-questions, which gives a relative frequency of around 0.1. Purpose,on the other hand, has only seven occurrences in our data collection, six of which beingaddressed by one or more questions, which gives a relative frequency of 0.857. Reasonand evidence both have only four occurrences in the collection, three of which havebeen addressed by one or more questions. Consequence even has a relative frequencyof 1.000 The table shows that if we address the problem of answer selection for why- questions as a discourse analysis task, the range of relation types that can lead toan answer is broad and should not be implemented too rigidly.
In section 3.1, we pointed out that our data collection may not be fully repre- sentative of a user’s information need, due to our elicitation method using a closeddocument set. The relation types in table 5 confirm that assumption to some extent:the presence of relation types such as means and condition suggests that the subjectsin some cases formulated why-questions whereas they would have formulated how-or when-questions in case of an actual information need. A question-answer pair likeWhy could FK-506 revolutionize the organ transplantation field? - Because it reducesharmful side effects and rejection rates, whereas the text reads FK-506 could revolu-tionize the transplantation field by reducing harmful side effects exemplifies this.
If we want to know our system’s performance on why-questions that are repre- sentative for a user’s information need, we are interested in those questions whoseanswers can be found through a ‘core-why relation’ like cause and reason.
If we only consider the relation types that have relative frequency higher than or equal to 0.5, we see that these relation types are in general closer to the concept ofreason as general answer type of why-questions (Verberne, 2006) than the relationtypes with a relative frequency lower than 0.5. We also see that the most frequentanswer types that we defined for question analysis (see section 1) come back in thisset of relation types. Purpose and reason, as defined by Carlson and Marcu (2001),correspond to our definition of the answer type motivation (Verberne et al., 2006a).
Carlson and Marcu (2001)’s consequence, result and cause relations can, based ontheir definitions, be grouped together as our answer type cause.
We investigated to what extent the performance of our system depends on the type of relation that leads from question topic to the reference answer. For this purpose, wesplit the relation types found in two categories: - Relation types that are conceptually close of the general answer type rea- son (‘core-why relations’): Purpose, Consequence, Evidence, Reason, Result,Explanation-argumentative and Cause. These relation types all have a relativefrequency higher than 0.5 for why-questions.
- Relation types that are less applicable to why-questions (‘non-why relations’): Means, Condition, Interpretation, Circumstance, Elaboration, Sequence, Listand Problem-Solution.
We considered the set of 207 questions for which the topic corresponds to the nu-cleus of a relation (thereby excluding the 74 questions whose topic or answer is un-supported, or where the RST relation does not lead to an answer) and measured oursystem’s recall on this set of questions. This is 77.5% — which is higher than thetotal recall of 51.2% because we excluded the majority of problematic cases. We thensplit the set of 207 questions into one set of questions whose answers can be foundthrough a core-why relation (130 questions), and one set of questions that correspondto a non-why relation (77 questions) and ran our system on both these sets. For thecore-why relation types, we found a system recall of 88.5% and for the non-why rela-tion types a system recall of 60.3%. Moreover, we found that the remaining 11.5% forthe core-why relation types suffer from lexical matching problems (see section 5.2) in-stead of procedural problems: for 100% of these questions, the satellite of the relationis a correct answer. For the non-why relation types, this is 85.9%.
Another problem of our data collection method, is that the questions formulated by the readers of the text (in particular the questions relating to core-why relations) willprobably be influenced by the same linguistic cues that are used by the annotators thatbuilt the RST structures: cue phrases (like because denoting an explanation relation)and syntactic constructions (like infinite clauses denoting a purpose relation). This isan unwelcome correlation, since in a working QA system users will not have access to the documents. Future work should indicate to what extent questions representinga real information need refer to why-relations in the RST structure.
6. Conclusions
We created a method for why-QA that is based on discourse structure and relations.
The main idea of our approach is that the propositions of a question topic and itsanswer are both represented by a text span in the source text, and that an RST relationholds between these spans. A why-question can then be answered by matching itstopic to a span in the RST tree and selecting the related span as answer.
We first investigated the possible contribution of the current RST approach to why- QA by performing a manual analysis of our set of 336 questions and answers collectedthrough elicitation from native speakers and connected to seven RST-annotated texts.
From the evaluation of our manual analysis, we concluded that for 58.0% of our why-questions, an RST relation holds between the text span corresponding to the questiontopic and the text span corresponding to the answer.
We implemented this method for discourse-based why-QA using the RST Tree- bank as document collection. Our system obtains a recall of 53.3% (91.8% of themanual score) with a MRR of 0.662.
In section 5.1, we conclude from the analysis of procedure shortcomings that there is a subset of why-questions (22.0%) that cannot be answered by a QA system thatexpects the topic of an input question to be present and identifiable in a closed docu-ment collection. For these questions, either the topic or the user-formulated answer isnot or only implicitly supported by the corresponding source text, which means thatworld knowledge is necessary for answering these questions. Furthermore, there isa further subset of why-questions (16.4%) that cannot be answered by a system thatuses RST structure following the approach we proposed. For these questions, thereis no RST relation between the span corresponding to the question topic and the spancorresponding to its answer. A third subset (3.6%) of problematic questions containsthose questions for which RST leads to an unsuitable or incomplete answer. Together,this means that 42.0% of why-questions cannot be answered following the suggestedapproach. Thus, the maximum recall that can be achieved with this method is 58.0%.
If we discard the questions that require world knowledge, maximum recall would be73.9%. An even higher performance can be achieved if we would only consider thosequestions that refer to core-why relations in the text like cause and reason.
In the near future we will focus our research on three topics. Firstly, we will investigate the why-questions (16.4% of the questions in our collection) where bothtopic and answer are supported by the source text, but where there is no RST relationbetween the span representing the question topic span and the answer span. We thinkthat other types of linguistic analysis, or different exploitation of the RST structurecan help for answering these questions.
Secondly, we aim to create and annotate a test corpus connected to why-questions that originate from real users’ information needs, based on the why-questions collectedfor the Webclopedia project (Hovy et al., 2002). With this set, we will investigate firstto what extent questions representing real information needs refer to why-relations ina document’s RST structure and second what the performance of our method is onsuch a set of questions.
Thirdly, we should note that in a future application of why-QA using RST, the system will not have access to a manually annotated corpus—it has to deal with au-tomatically annotated data. We assume that automatic RST annotations will be lesscomplete and less precise than the manual annotations are. As a result of that, perfor-mance would decline if we were to use automatically created annotations. Some workhas been done on automatically annotating text with discourse structure. Promising isthe done work by Marcu and Echihabi (2001), Soricut and Marcu (2003) and Huongand Abeysinghe (2003). We plan to investigate to what extent we can achieve partialautomatic discourse annotations that are specifically equipped to finding answers towhy-questions. We think we can make such annotations feasible if we focus on theinformation that is needed for answering why-questions, based on the knowledge thatwe obtained from the work described in the present paper.
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