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Keyword Extraction and Clustering for DocumentRecommendation in Conversations

Maryam Habibi and Andrei Popescu-Belis

Abstract—This paper addresses the problem of keyword extrac-tion from conversations, with the goal of using these keywords toretrieve, for each short conversation fragment, a small numberof potentially relevant documents, which can be recommended toparticipants. However, even a short fragment contains a variety ofwords, which are potentially related to several topics; moreover,using an automatic speech recognition (ASR) system introduceserrors among them. Therefore, it is difficult to infer preciselythe information needs of the conversation participants. We firstpropose an algorithm to extract keywords from the output of anASR system (or a manual transcript for testing), which makesuse of topic modeling techniques and of a submodular rewardfunction which favors diversity in the keyword set, to match thepotential diversity of topics and reduce ASR noise. Then, wepropose a method to derive multiple topically-separated queriesfrom this keyword set, in order to maximize the chances ofmaking at least one relevant recommendation when using thesequeries to search over the English Wikipedia. The proposedmethods are evaluated in terms of relevance with respect toconversation fragments from the Fisher, AMI, and ELEA con-versational corpora, rated by several human judges. The scoresshow that our proposal improves over previous methods thatconsider only word frequency or topic similarity, and representsa promising solution for a document recommender system to beused in conversations.

Keywords—Keyword extraction, topic modeling, meeting analysis,information retrieval, document recommendation.

I. INTRODUCTION

HUMANS are surrounded by an unprecedented wealthof information, available as documents, databases, or

multimedia resources. Access to this information is condi-tioned by the availability of suitable search engines, but evenwhen these are available, users often do not initiate a search,because their current activity does not allow them to do so,or because they are not aware that relevant information isavailable. We adopt in this paper the perspective of just-in-timeretrieval, which answers this shortcoming by spontaneouslyrecommending documents that are related to users’ currentactivities. When these activities are mainly conversational, forinstance when users participate in a meeting, their informationneeds can be modeled as implicit queries that are constructed inthe background from the pronounced words, obtained through

Copyright (c) 2013 IEEE. Personal use of this material is permitted.However, permission to use this material for any other purposes must beobtained from the IEEE by sending a request to pubs-permissions@ieee.org.

Idiap Research Institute and Ecole Polytechnique Federalede Lausanne (EPFL), Rue Marconi 19, CP 592, 1920 Mar-tigny, Switzerland. e-mail: maryam.habibi@idiap.ch andandrei.popescu-belis@idiap.ch.

real-time automatic speech recognition (ASR). These implicitqueries are used to retrieve and recommend documents fromthe Web or a local repository, which users can choose to inspectin more detail if they find them interesting.

The focus of this paper is on formulating implicit queriesto a just-in-time-retrieval system for use in meeting rooms.In contrast to explicit spoken queries that can be made incommercial Web search engines, our just-in-time-retrieval sys-tem must construct implicit queries from conversational input,which contains a much larger number of words than a query.For instance, in the example discussed in Section V-B4 below,in which four people put together a list of items to help themsurvive in the mountains, a short fragment of 120 secondscontains about 250 words, pertaining to a variety of domains,such as ‘chocolate’, ‘pistol’, or ‘lighter’. What would then bethe most helpful 3–5 Wikipedia pages to recommend, and howwould a system determine them?

Given the potential multiplicity of topics, reinforced bypotential ASR errors or speech disfluencies (such as ‘whisk’in this example), our goal is to maintain multiple hypothesesabout users’ information needs, and to present a small sampleof recommendations based on the most likely ones. Therefore,we aim at extracting a relevant and diverse set of keywords,cluster them into topic-specific queries ranked by importance,and present users a sample of results from these queries.The topic-based clustering decreases the chances of includingASR errors into the queries, and the diversity of keywordsincreases the chances that at least one of the recommendeddocuments answers a need for information, or can lead to auseful document when following its hyperlinks. For instance,while a method based on word frequency would retrieve thefollowing Wikipedia pages: ‘Light’, ‘Lighting’, and ‘Light MyFire’ for the above-mentioned fragment, users would prefer aset such as ‘Lighter’, ‘Wool’ and ‘Chocolate’.

Relevance and diversity can be enforced at three stages:when extracting the keywords; when building one or severalimplicit queries; or when re-ranking their results. The first twoapproaches are the focus of this paper. Our recent experimentswith the third one, published separately [1], show that re-ranking of the results of a single implicit query cannot improveusers’ satisfaction with the recommended documents. Previousmethods for formulating implicit queries from text (discussedextensively in Section II-B) rely on word frequency or TFIDFweights to rank keywords and then select the highest rankingones [2], [3]. Other methods perform keyword extraction byusing topical similarity [4]–[6], but do not set a topic diversityconstraint.

In this paper, we introduce a novel keyword extractiontechnique from ASR output, which maximizes the coverage of

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potential information needs of users and reduces the numberof irrelevant words. Once a set of keywords is extracted, it isclustered in order to build several topically-separated queries,which are run independently, offering better precision than alarger, topically-mixed query. Results are finally merged into aranked set before showing them as recommendations to users.

The paper is organized as follows. In Section II-A wereview existing just-in-time-retrieval systems and the policiesthey use for query formulation. In Section II-B we discussprevious methods for keyword extraction. In Section III wedescribe the proposed technique for implicit query formulation,which relies on a novel topic-aware diverse keyword extractionalgorithm (III-A) and a topic-aware clustering method (III-B).Section IV introduces the data and our method for comparingthe relevance of sets of keywords or recommended documentsusing crowdsourcing. In Section V we present and discussthe experimental results on keyword extraction and documentrecommendation. We also exemplify the results on one con-versation fragment given in Appendix A.

II. STATE OF THE ART: JUST-IN-TIME RETRIEVAL ANDKEYWORD EXTRACTION

Just-in-time retrieval systems have the potential to bringa radical change in the process of query-based informationretrieval. Such systems continuously monitor users’ activitiesto detect information needs, and pro-actively retrieve relevantinformation. To achieve this, the systems generally extractimplicit queries (not shown to users) from the words thatare written or spoken by users during their activities. In thissection, we review existing just-in-time-retrieval systems andmethods used by them for query formulation. In particular,we will introduce our Automatic Content Linking Device(ACLD) [7], [8], a just-in-time document recommendationsystem for meetings, for which the methods proposed in thispaper are intended. In II-B, we discuss previous keywordextraction techniques from a transcript or text.

A. Query Formulation in Just-in-Time Retrieval SystemsOne of the first systems for document recommendation,

referred to as query-free search, was the Fixit system [9], anassistant to an expert diagnostic system for the products of aspecific company (fax machines and copiers). Fixit monitoredthe state of the user’s interaction with the diagnostic system,in terms of the positions in a belief network built from therelations among symptoms and faults, and ran backgroundsearches on a database of maintenance manuals to provideadditional support information related to the current state.

The Remembrance Agent [10], [11], another early just-in-time retrieval system, is closer in concept to the system con-sidered in this paper. The Remembrance Agent was integratedinto the Emacs text editor, and ran searches at regular timeintervals (every few seconds) using a query that was based onthe latest words typed by the user, for instance using a buffer of20–500 words ranked by frequency. The Remembrance Agentwas extended to a multimodal context under the name ofJimminy, a wearable assistant that helped users with takingnotes and accessing information when they could not use

a standard computer keyboard, e.g. while discussing withanother person [12]. Using TFIDF for keyword extraction,Jimminy augmented these keywords with features from othermodalities, for example the user’s position and the name oftheir interlocutor(s).

The Watson just-in-time-retrieval system [13] assisted userswith finding relevant documents while writing or browsing theWeb. Watson built a single query based on a more sophis-ticated mechanism than the Remembrance Agent, by takingadvantage of knowledge about the structure of the writtentext, e.g. by emphasizing the words mentioned in the abstractor written with larger fonts, in addition to word frequency.The Implicit Queries (IQ) system [14], [15] generated context-sensitive searches by analyzing the text that a user is readingor composing. IQ automatically identified important wordsto use in a query using TFIDF weights. Another query-freesystem was designed for enriching television news with articlesfrom the Web [16]. Similarly to IQ or Watson, queries wereconstructed from the ASR using several variants of TFIDFweighting, and considering also the previous queries made bythe system.

Other real-time assistants are conversational: they interactwith users to answer their explicit information needs or toprovide recommendations based on their conversation. Forinstance, Ada and Grace1 are twin virtual museum guides [17],which interact with visitors to answer their questions, suggestexhibits, or explain the technology that makes them work.A collaborative tourist information retrieval system [18], [19]interacts with tourists to provide travel information such asweather conditions, attractive sites, holidays, and transporta-tion, in order to improve their travel plans. MindMeld2 is acommercial voice assistant for mobile devices such as tablets,which listens to conversations between people, and showsrelated information from a number of Web-based informationsources, such as local directories. MindMeld improves theretrieval results by adding the users’ location information tothe keywords of conversation obtained using an ASR system.As far as is known, the system uses state-of-the-art method forlanguage analysis and information retrieval [20].

In collaboration with other researchers, we have designedthe Automatic Content Linking Device (ACLD) [7], [8] whichis a just-in-time retrieval system for conversational environ-ments, especially intended to be used jointly by a small groupof people in a meeting. The system constantly listens to themeeting and prepares implicit queries from words recognizedthrough ASR. A selection of the retrieved documents isrecommended to users. Before the solutions proposed in thispaper, the ACLD modeled users’ information needs as a set ofkeywords extracted at regular time intervals, by matching theASR against a list of keywords fixed before the meeting. Weshowed that this method outperforms the use of the entire set ofwords from a conversation fragment as an implicit query [21].Moreover, experiments with the use of semantic similaritybetween a conversation fragment and documents as a criterionfor recommendation have shown that, although this improves

1See http://ict.usc.edu/prototypes/museum-guides/.2See http://www.expectlabs.com/mindmeld/.

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relevance, its high computation cost makes it unpractical forjust-in-time retrieval from a large repository [22, 4.12].

These findings motivated us to design an innovative keywordextraction method for modeling users’ information needs fromconversations. As mentioned in the introduction, since evenshort conversation fragments include words potentially pertain-ing to several topics, and the ASR transcript adds additionalambiguities, a poor keyword selection method leads to non-informative queries, which often fail to capture users’ infor-mation needs, thus leading to low recommendation relevanceand user satisfaction. The keyword extraction method proposedhere accounts for a diversity of hypothesized topics in adiscussion, and is accompanied by a clustering technique thatformulates several topically-separated queries.

B. Keyword Extraction MethodsNumerous methods have been proposed to automatically

extract keywords from a text, and are applicable also totranscribed conversations. The earliest techniques have usedword frequencies [2] and TFIDF values [3], [23] to rankwords for extraction. Alternatively, words have been rankedby counting pairwise word co-occurrence frequencies [24].These approaches do not consider word meaning, so they mayignore low-frequency words which together indicate a highly-salient topic. For instance, the words ‘car’, ‘wheel’, ‘seat’, and‘passenger’ occurring together indicate that automobiles are asalient topic even if each word is not itself frequent [25].

To improve over frequency-based methods, several ways touse lexical semantic information have been proposed. Semanticrelations between words can be obtained from a manually-constructed thesaurus such as WordNet, or from Wikipedia,or from an automatically-built thesaurus using latent topicmodeling techniques such as LSA, PLSA, or LDA. For in-stance, keyword extraction has used the frequency of all wordsbelonging to the same WordNet concept set [4], while theWikifier system [5] relied on Wikipedia links to computeanother substitute to word frequency. Hazen also applied topicmodeling techniques to audio files [26]. In another study, heused PLSA to build a thesaurus, which was then used to rankthe words of a conversation transcript with respect to eachtopic using a weighted point-wise mutual information scoringfunction [27]. Moreover, Harwath and Hazen utilized PLSAto represent the topics of a transcribed conversation, and thenranked words in the transcript based on topical similarity tothe topics found in the conversation [6]. Similarly, Harwathet al. extracted the keywords or key phrases of an audio fileby directly applying PLSA on the links among audio framesobtained using segmental dynamic time warping, and thenusing mutual information measure for ranking the key conceptsin the form of audio file snippets [28]. A semi-supervised latentconcept classification algorithm was presented by Celikyil-maz and Hakkani-Tur using LDA topic modeling for multi-document information extraction [29].

To consider dependencies among selected words, word co-occurrence has been combined with PageRank [30], and addi-tionally with WordNet [31], or with topical information [32].For instance, Riedhammer et al. considered the dependencies

among surrounding words by merging n-gram information ob-tained from WordNet with word frequency, in order to extractkeywords from a meeting transcript [33]. To reduce the effectof noise in the meeting environments, this method removed alln-grams which appear only once or are represented by longern-grams with the same frequencies. However, as shown empiri-cally in [30], [32] such approaches have difficulties modelinglong-range dependencies between words related to the sametopic. In another study, part-of-speech information and wordclustering techniques were used for keyword extraction [34],while later this information was added to TFIDF so as toconsider both word dependency and semantic information [35].In a recent paper, a word clustering technique was introducedby [36] based on the word2vec vector space representation ofa word in which the dependencies between each word andits surrounding words are modeled using a neural networklanguage model [37], [38]. However, although they consideredtopical similarity and dependency among words, the abovemethods did not explicitly reward diversity and therefore mightmiss secondary topics in a conversation fragment.

Supervised machine learning methods have been used tolearn models for extracting keywords. This approach wasfirst introduced by Turney [39], who combined heuristic ruleswith a genetic algorithm. Other learning algorithms such asNaive Bayes [40], Bagging [41], or Conditional Random Fields[42] have been used to improve accuracy. These approaches,however, rely on the availability of in-domain training data, andthe objective functions they use for learning do not considerthe diversity of keywords.

III. FORMULATION OF IMPLICIT QUERIES FROMCONVERSATIONS

We propose a two-stage approach to the formulation ofimplicit queries. The first stage is the extraction of keywordsfrom the transcript of a conversation fragment for whichdocuments must be recommended, as provided by an ASRsystem (Subsection III-A). These keywords should cover asmuch as possible the topics detected in the conversation, andif possible avoid words that are obviously ASR mistakes. Thesecond stage is the clustering of the keyword set in the formof several topically-disjoint queries (Subsection III-B).

A. Diverse Keyword ExtractionWe propose to take advantage of topic modeling techniques

to build a topical representation of a conversation fragment,and then select content words as keywords by using topicalsimilarity, while also rewarding the coverage of a diverse rangeof topics, inspired by recent summarization methods [43],[44]. The benefit of diverse keyword extraction is that thecoverage of the main topics of the conversation fragmentis maximized. Moreover, in order to cover more topics, theproposed algorithm will select a smaller number of keywordsfrom each topic. This is desirable for two reasons. First, aswe will see in Section III-B on keyword clustering, this willlead to more dissimilar implicit queries, thus increasing thevariety of retrieved documents. Second, if words which arein reality ASR noise can create a main topic in the fragment,

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then the algorithm will choose a smaller number of these noisykeywords compared to algorithms which ignore diversity.

The proposed method for diverse keyword extraction pro-ceeds in three steps, represented schematically in Figure 1 (afirst version of this method appeared in [45]). First, a topicmodel is used to represent the distribution of the abstract topicz for each word w noted p(z|w) as depicted in Figure 1. Theabstract topics are not pre-defined manually but are representedby latent variables using a generative topic modeling technique.These topics occur in a collection of documents – preferably,one that is representative of the domain of the conversations.Second, these topic models are used to determine weights forthe abstract topics in each conversation fragment representedby βz . These steps are described in Section III-A1. Finally,the keyword list W = {w1, ..., wk} which covers a maximumnumber of the most important topics are selected by reward-ing diversity, using an original algorithm introduced in thissections.

Topic modeling information

Transcript

Determine the weights of conversation topics

Extract the best k keywords that cover all the main topics with

high probability

},...,{ 1 kwwW �

z�

)|( 1wzp

z

(1)

(2)

(3)

z

z�)|( nwzp

Fig. 1. The three steps of the proposed keyword extraction method: (1) topicmodeling, (2) representation of the main topics of the transcript, and (3) diversekeyword selection.

1) Modeling Topics in Conversations: Topic models suchas Probabilistic Latent Semantic Analysis (PLSA) or LatentDirichlet Allocation (LDA) [46] can be used as off-linetopic modeling techniques to determine the distribution overthe topic z of each word w, noted p(z|w), from a largeamount of training documents. LDA implemented in the Mallettoolkit [47] is used in this paper because it does not suffer fromthe overfitting issue of PLSA, as discussed in [46].

When a conversation fragment is considered for keywordextraction, its topics are weighted, each by βz which isobtained by averaging over all probabilities p(z|wi) of the N

words wi spoken in the fragment.

βz =1

N

∑1≤i≤N

p(z|wi) (1)

2) Diverse Keyword Extraction Problem: The goal of thekeyword extraction technique with maximal topic coverage isformulated as follows. If a conversation fragment t mentions aset of topics Z, and each word w from the fragment t can evokea subset of the topics in Z, then the goal is to find a subsetof k unique words S ⊆ t, with |S| = k, which maximizes thenumber of covered topics.

This problem is an instance of the maximum coverage prob-lem, which is known to be NP-hard. If the coverage function issubmodular and monotone nondecreasing3, a greedy algorithmcan find an approximate solution guaranteed to be within(1− 1

e ) = 0.63 of the optimal solution in polynomial time [48].To achieve our goal, we define the contribution of a topic

z with respect to each set of words S ⊆ t of size k bysumming over all probabilities p(z|w) of the words in the set.Afterward, we propose a reward function, for each set S andtopic z, to model the contribution of the set S to the topicz. Finally, we select one of the sets S ⊆ t which maximizesthe cumulative reward values over all the topics. The wholeprocedure is formalized below.

3) Definition of a Diverse Reward Function: We introducerS,z , the contribution towards topic z of the keyword set Sselected from the fragment t:

rS,z =∑w∈S

p(z|w) (2)

We propose the following reward function for each topic,where βz represents the weight of topic z over all the wordsof the fragment to assign a higher weight to topics withhigher value and λ is a parameter between 0 and 1. Thisis a submodular function with diminishing returns when rS,zincreases, as proved in Appendix B.

f : rS,z → βz · rλS,z (3)

Finally, the keyword set S ⊆ t, is chosen by maximizing thecumulative reward function over all the topics, formulated asfollows:

R(S) =∑z∈Z

βz · rλS,z (4)

In this equation, if candidate keywords which are in fact ASRerrors (insertions or substitutions) are associated with topicswith lower βz , as is most often the case, the probability oftheir selection by the algorithm will reduced, because theircontribution to the reward will be small.

Since the class of submodular functions is closed under non-negative linear combinations [48], R(S) is a monotone non-decreasing submodular function. If λ = 1, the reward functionis linear and only measures the topical similarity of wordswith the main topics of t. However, when 0 < λ < 1, as

3A function F is submodular if ∀A ⊆ B ⊆ T \ t, F (A + t) − F (A) ≥F (B + t) − F (B) (diminishing returns) and is monotone nondecreasing if∀A ⊆ B, F (A) ≤ F (B).

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soon as a word is selected from a topic, other words from thesame topic start having diminishing gains as candidates forselection.Therefore, decreasing the value of λ increases thediversity constraint, which increases the chance of selectingkeywords from secondary topics. As these words may reducethe overall relevance of the keyword set, it is essential to finda value of the hyper-parameter λ which leads to the desiredbalance between relevance and diversity in the keyword set.

According to a different perspective, the definition of R(S)in Equation 4 can be seen as the dot product in the topic spacebetween the weights βz obtained from the topic probabilitiesgiven the fragment t and the reward function over the sumof topic probabilities rλS,z with a scaling exponent λ andidentical coefficients over all topics. However, despite whatthis apparent similarity suggests, the use of cosine similarityfor R(S) would not lead to an appropriate definition becauseit would not provide a monotone non-decreasing submodularfunction. Indeed, if vector length normalization is introducedin R(S), for cosine similarity, then we can show that R(S) isno longer monotone submodular, e.g. on the second examplein the following subsection.

4) Examples: We will illustrate the motivation for ourdefinition of R(S) on the following example. Let us considera situation with four words w1, w2, w3, w4. The goal is toselect two of them as keywords which cover the main topicspresented by these four words. Suppose that each word can berelated to two topics z1 and z2. The probability of topic z1 forwords w1 and w2 is 1, and for words w3 and w4 it is zero, andvice versa for topic z2. Therefore, βz1 = βz2 = 0.5. For twosample sets S1 = {w1, w2} and S2 = {w1, w3} the cumulativerewards are respectively R(S1) = 0.5 · (1 + 1)λ + 0.5 · 0λand R(S2) = 0.5 · 1λ + 0.5 · 1λ. Since R(S1) ≤ R(S2) for0 < λ < 1, the keyword set S2 which covers two main topicsis selected. If λ = 1 then the cumulative reward for the twosets S1 and S2 is equal, which does not guarantee to selectthe set which covers both topics.

The example above has the desirable values of R(S) re-gardless of whether the dot product or the cosine similarityare used for the definition of R(S) in Equation 4. However,this is not always the case. In the example shown in Table I (towhich we will refer again below), if we consider A = {w5},B = {w3, w5} and λ = 0.75, then A ⊆ B but R(A) =0.76 > R(B) = 0.70 if cosine similarity is used, hence thisversion of R(S) would not be monotone non-decreasing. Ifwe add keyword w4 to both keyword sets A and B, thenR(A∪ {w4})−R(A) = 0.02 < R(B∪ {w4})−R(B) = 0.09,hence R(S) would neither have the diminishing returns prop-erty, if cosine similarity was used.

5) Comparison with the Function Used for Summarization:We are inspired by recent work on extractive summarizationmethods [43] [44], to define a monotone submodular functionfor keyword extraction which maximizes the number of cov-ered topics. This work proposed a square root function as areward function for the selection of sentences, to cover themaximum number of concepts of a given document. Note thatin their work, this function rewards diversity by increasing thegain of selecting a sentence including a concept that was notyet covered by a previously selected sentence. However, we

propose a reward function for diverse selection of keywordsas a power function with a scaling exponent between 0 and1, and a coefficient corresponding to the weight of each topicconveyed in the fragment. Therefore, we generalize the squareroot function and the constant coefficient equal to one for allconcepts which is proposed by [43] and [44].

In our reward function, the scaling exponent between 0 and 1applies diversity by decreasing the reward of keyword selectionfrom a topic when the number of keywords representingthat topic increases, and increasing the reward of selectingkeywords from the topics which are not covered yet. In contrastto the summarization techniques proposed by [43] [44] whichadd a separate term for considering the relevance and thecoverage of the main concepts of the given text by summarysentences, we used a coefficient corresponding to the weightof topics conveyed in the fragment.

6) Finding the Optimal Keyword Set: To maximize R(S) inpolynomial time under the cardinality constraint of |S| = k wepresent a greedy algorithm shown as Algorithm 1. In the firststep of the algorithm, S is empty. At each step, the algorithmselects one of the unselected words from the conversation frag-ment w ∈ t\S which has the maximum similarity to the maintopics of the conversation fragment and also maximizes thecoverage of the topics with respect to the previously selectedkeywords in S. This is as h(w, S) =

∑z∈Z βz[p(z|w)+rS,z]λ,

where p(z|w) is the contribution to topic z by word w ∈ t \Swhich is added to the contribution of the topic z in the setS. The algorithm updates the set S by adding one of thewords w ∈ t \ S to the set S which maximizes h(w, S).This procedure continues until reaching k keywords from thefragment t.

Input : a given text t, a set of topics Z, the number ofkeywords k

Output: a set of keywords SS ← ∅;while |S| ≤ k do

S ← S ∪ {argmaxw∈t\S(h(w, S))whereh(w, S) =

∑z∈Z βz[p(z|w) + rS,z]

λ;endreturn S;

Algorithm 1: Diverse keyword extraction.

7) Illustration of the Greedy Algorithm: We will exemplifythe mechanism of the proposed algorithm using a simpleexample. Let us consider a conversation fragment with fivewords, each represented by four topics. The distributions oftopics for each word are given in Table I. The topics are thusweighted as follows: βz1 = 0.42, βz2 = 0.20, βz3 = 0.06, andβz4 = 0.32. We run the algorithm to extract two keywordsout of five for λ ∈ {.75, 1}. In other words, λ = 1 selectswords based on their topical similarity to the main topics ofthe conversation, and λ = .75 considers both topical diversityand similarity for keyword extraction.

Initially S is empty. The reward values, h(w, S = ∅), for allwords and λ ∈ {.75, 1} are shown in Table II. In the first stepof the algorithm, w1 (the best representative of topic z1) is

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TABLE I. SAMPLE INPUT TO THE GREEDY ALGORITHM

Words p(z1|·) p(z2|·) p(z3|·) p(z4|·)w1 1.00 0.00 0.00 0.00w2 0.90 0.00 0.10 0.00w3 0.00 0.00 0.20 0.80w4 0.10 0.90 0.00 0.00w5 0.10 0.10 0.00 0.80

added to the set S for both values of λ. In the second step, theh(w, {w1}) values are computed for the remaining unselectedwords, and both λ values, as shown in Table II. According tothese values, λ = 1 selects w2 as the second word from thetopic z1. However, λ = .75 selects w5 as the second keyword(the best representative of topic z4), the second main topic ofthe conversation fragment, because it rewards topical diversityin the keyword set.

TABLE II. THE h(w, S) VALUES CALCULATED USING ALGORITHM 1TO SELECT TWO KEYWORDS OUT OF 5 WORDS FOR λ = .75 AND 1.

λ = 1 λ = .75Words h(·, ∅) h(·, {w1}) h(·, ∅) h(·, {w1})w1 0.420 – 0.420 –w2 0.384 0.804 0.398 0.690w3 0.268 0.688 0.288 0.708w4 0.222 0.642 0.259 0.635w5 0.318 0.738 0.380 0.757

B. Keyword ClusteringThe diverse set of extracted keywords is considered to

represent the possible information needs of the participantsto a conversation, in terms of the notions and topics that arementioned in the conversation. To maintain the diversity oftopics embodied in the keyword set, and to reduce the noisyeffect of each information need on the others, this set mustbe split into several topically-disjoint subsets. Each subsetcorresponds then to an implicit query that will be sent toa document retrieval system. These subsets are obtained byclustering topically-similar keywords, as follows.

Clusters of keywords are built by ranking keywords for eachmain topic of the fragment. The keywords are ordered for eachtopic by decreasing values of βz · p(z|w). Moreover, in eachcluster, only the keywords with a βz · p(z|w) value higher thana threshold (0.01 in the current setting) are kept for each topicz. Note that a given keyword can appear in more than onecluster. Following this ordering criterion, keywords with highvalue of p(z|w) (i.e. more representative of the topic) will beranked higher in the cluster of topic z and these keywords willbe selected from the topics with high value of βz . Afterward,clusters themselves are ranked based on their βz values.

C. From Keywords to Document RecommendationsAs a first idea, one implicit query can be prepared for

each conversation fragment by using as a query all keywordsselected by the diverse keyword extraction technique. However,to improve the retrieval results, multiple implicit queries can beformulated for each conversation fragment, with the keywordsof each cluster from the previous section, ordered as above

(because the search engine used in our system is not sensitiveto word order in queries).

In experiments with only one implicit query per conver-sation fragment, the document results corresponding to eachconversation fragment were prepared by selecting the first ddocument retrieval results of the implicit query.

In the experiments below with multiple implicit queries,the recommendation lists were prepared by selecting the firstdocument retrieval results of each implicit query and thenranking documents based on the topical similarity of theircorresponding queries to the conversation fragment. This isa baseline algorithm, but we describe an improved rankingmethod in [1].

IV. DATA AND EVALUATION METHODS

Our proposals were tested on three conversational corpora,the Fisher Corpus [49], the AMI Meeting Corpus [50], andthe ELEA Corpus [51]. The relevance of the keywords wasassessed by designing a comparison task and averaging severaljudgments obtained by crowdsourcing this task through theAmazon Mechanical Turk (AMT) platform. In addition, the α-NDCG measure [52] was used to measure topic diversity in thelist of keywords. Afterward, the quality of implicit queries wasassessed by estimating (again with human judges recruited viaAMT) the relevance of the documents that were retrieved whensubmitting these queries to the Lucene search engine overthe English Wikipedia and merging the results as explainedabove. Here, the conversational data came only from the ELEACorpus, which offers clearer criteria for assessing the relevanceof recommendations than the Fisher and AMI Corpora. Wenow describe the three corpora and the data extracted fromthem, as well as the evaluation methods for each task.

A. Conversational Corpora Used for ExperimentsThe Fisher Corpus [49] contains about 11,000 topic-labeled

telephone conversations, on 40 pre-selected topics (one perconversation). In our experiments, we used the manual refer-ence transcripts available with the corpus. We created a topicmodel using the Mallet [47] implementation of LDA, over twothirds of the Fisher Corpus, given the sufficient number ofsingle-topic documents, fixing the number of abstract topicsat 40. The remaining data was used to build 11 artificialconversation fragments (1-2 minutes long) for testing, byconcatenating 11 times three fragments about three differenttopics.

The AMI Meeting Corpus [50] contains conversations ondesigning remote controls, in series of four scenario-basedmeetings each, for a total of 171 meetings. Speakers were notconstrained to talk about a single topic throughout a meeting,hence these transcripts are multi-topic4. Since the number ofmeetings in the AMI Corpus is not large enough for buildingtopic models with LDA, we used a subset of the EnglishWikipedia with 124,684 articles. Following several previousstudies [53], [54], we fixed the number of topics at 100.

4The annotation of “episodes” that is provided with the AMI Corpusconsiders goal-based rather than topic-based episodes which are not usablehere.

7

(a) (b)

Please select one of the following options:1. Image (a) represents the conversation fragment better than (b).2. Image (b) represents the conversation fragment better than (a).3. Both (a) and (b) offer a good representation of the conversation.4. None of (a) and (b) offer a good representation of the conversation.

Fig. 2. Example of an evaluation task based on an AMI discussion about the impact of the features of a remote control on its sales. The word clouds weregenerated using WordleTM from the lists produced in this example by: (a) the diverse keyword technique with λ =0.75, and (b) a topical similarity method.The latter over-represents the topic ‘color’ by selecting three words related to it, but misses other topics such as ‘remote control’, ‘losing a device’ and ‘buyinga device’ which are also representative of the fragment.

We selected for testing 8 conversation fragments, each 2-3minutes long, from the AMI Corpus. We used both manual andASR transcripts of these fragments. The ASR transcripts weregenerated by the AMI real-time ASR system for meetings [55],with an average word error rate (WER) of 36%.

In addition, for experimenting with a variable range of WERvalues, we applied to the AMI manual transcripts simulatedASR noise (deletion, insertion and substitution). Namely, werandomly deleted words, or added new words, or substitutedwords by other words, in a systematic manner, i.e. all oc-currences of a given word type were altered. We randomlyselected the word types to be deleted or substituted, as well asthe words to be inserted (from the vocabulary of the EnglishWikipedia), using a variable percentage of noise from 5% to50%. This simulation technique is actually more challenging toour application than actual ASR errors, which are not createdrandomly, and tend to spare long content words because theyhave fewer homophones.

The ELEA Corpus (Emergent LEader Analysis) [51] con-sists of approximately ten hours of recorded and transcribedmeetings, in English and French. Each meeting is a role playgame in which participants are supposed to be survivors of anairplane crash, and must rank a list of 12 items with respectto their utility for surviving in the mountain until they arerescued. In our experiments, we considered 5 conversations inEnglish, of about 15 minutes each, and divided their transcriptsinto 35 segments, of about two minutes each. Similarly to theAMI Corpus, the transcripts in the ELEA Corpus were notsufficient for topic modeling. Therefore, the same subset ofthe English Wikipedia as for the AMI Corpus was utilized tolearn topic models.

B. Evaluation Protocol and MetricsWe designed comparison tasks to evaluate the relevance of

extracted keywords and of recommended documents with re-spect to each conversation fragment. For the former evaluation,we compared the relevance (or representativeness) of two listsof keywords extracted from the same conversation fragment bytwo different extraction methods. We displayed the transcriptof the fragment to a human subject in a web browser, followed

below it by several control questions about its content, andthen by two lists of keywords (typically, nine keywords inour experiments). To improve readability, the keyword listswere presented using a word cloud representation generatedby the WordleTM tool5, in which the words ranked higherare emphasized in the cloud. The subjects had to read theconversation transcript, answer the control questions, and thendecide which keyword cloud better represented the content ofthe conversation fragment. The task is exemplified in Figure 2,without the control questions or the conversation transcript. Asimilar method was applied to compare recommended docu-ments, except that two lists of retrieved documents (typically,with seven items each) are shown instead of word clouds, andtheir potential utility as recommendations to the conversationparticipants is compared.

In both cases, comparative judgments were integrated overa large number of subjects and conversation fragments. To thisend, the tasks were crowdsourced via Amazon’s MechanicalTurk as “human intelligence tasks” or HITs. For each compar-ison task we recruited ten workers completing several HITs ina row, though with a limit to avoid fatigue. The average timespent per HIT was about 2.5 minutes. For qualification control,we only accepted workers with greater than 95% approval rate(i.e. 95% of the worker’s submitted HITs for previous taskshad been approved by their requesters) and with more than1000 approved HITs. We only kept answers from the workerswho answered correctly our control questions about each HIT.

Results obtained on the eight manual transcripts of theconversation fragments of the AMI Corpus (noted A–H) areshown in Table III, for the comparison of our proposal forkeyword extraction (noted D(.75) as explained in V-A) with amethod using only topic similarity (noted TS). The ten subjectscould choose between the four comparative answers presentedin Figure 2, which amount to: ‘X better than Y’, ‘Y better thanX’, ‘both good’, or ‘both bad’. No answers were rejected forthese HITs. The total counts for each answer and each HITfrom Table III indicate that subjects agreed strongly on certainanswers to some HITs (e.g. C, D, H) but disagreed on others(mainly A). The challenge is therefore to compute a reliable

5(http://www.wordle.net)

8

comparative score for two methods from such counts.

TABLE III. NUMBER OF ANSWERS FOR EACH OF THE FOUR OPTIONSOF THE EVALUATION TASK, FROM TEN JUDGES. THE 8 HITS (A THROUGH

H) COMPARE OUR DIVERSE KEYWORD EXTRACTION METHOD (D(.75))AND THE TOPICAL SIMILARITY (TS) ONE.

A B C D E F G HKeywords obtained by the topicalsimilarity method are more relevant

4 1 1 1 2 2 1 1

Keywords extracted by the diversetechnique (D(.75)) are more relevant

4 1 8 9 6 6 6 8

Both keyword lists are relevant 2 5 1 0 2 2 3 1Both keyword lists are irrelevant 0 3 0 0 0 0 0 0

We compute comparative relevance scores as follows, basedon a method that we proposed in a previous paper [21] forcomparing the search results of two document recommendersystems for meetings. First, we apply a qualification controlfactor to the human judgments, reducing the impact of workerswho disagree with the majority, using the Pearson correlationof one worker’s judgment with the average of all others, notedRv . We then factor out the impact of undecided HITs by usinga measure of the entropy of the answer distribution for eachHIT Tj . Finally, we average the results of all judgments andHITs to obtain a comparative score noted CSa for each setof answers a as shown in Eq. 5, where H is the number ofHITs and Xv(h, a) is the judgment of worker v for HIT h andanswer list a. When comparing two sets of answers, the sumof the two scores is 100%.

CSa =

(H∑h=1

(1− Th)∑Vv=1RvXv(h, a)∑V

v=1Rv

)/

H∑h=1

(1− Th)

(5)To evaluate the diversity of keyword sets, we use the α-

NDCG measure proposed for information retrieval [52], whichrewards a mixture of relevance and diversity, with equalweights when α = .5 as set here. We only apply α-NDCG tothe three-topic conversation fragments from the Fisher Corpus,because this is the only dataset with explicitly marked topics.The values are computed according to Eq. 6, where DCG isobtained by Eq. 7 and DCGideal is computed by reorderingwords in a way that maximizes DCG values in each rank. InEq. 7, T is the number of mono-topic dialogues included inthe conversation, rj,i−1 is the number of relevant words tothe mono-topic dialogue j until rank i, and J(wi, j) measuresthe relevance of the word w in rank i to the mono-topicdialogue j according to Eq. 8. We set the relevance of aword to a conversation topic at 1 when the keyword belongsto the corresponding fragment (one keyword may be relevantto several topics) as shown in Eq. 8. A higher α-NDCGvalue indicates that keywords from the set are more uniformlydistributed across the three topics.

α -NDCG[k] = DCG[k]/DCGideal (6)

DCG[k] =

k∑i=1

∑Tj=1 J(wi, j)(1− α)rj,i−1

log2(1 + i)(7)

J(wi, j) =

{1 if wi ∈ dialoguej0 otherwise

(8)

V. EXPERIMENTAL RESULTS

In this section, the diverse keyword extraction technique iscompared with two state-of-the-art methods, showing that ourproposal extracts more relevant keywords, which cover moretopics, and are less likely to be ASR errors. Then, we comparethe retrieval results of the implicit queries generated from thesekeyword lists, either applied entirely or by decomposing theminto topically-separated queries, again showing that the listsgenerated by our method outperform existing methods.

A. Evaluation of Keyword Extraction1) Selection of Configurations: We compare several versions

of the proposed diverse keyword extraction method (keywordsobtained using Algorithm 1) noted D(λ) for λ ∈ {.5, .75, 1},a method using only word frequency (noted WF, excludingstopwords), and a recent method based on topical similarity(noted TS) which does not enforce diversity [6]. In fact, D(1)coincides with TS.

The three values of λ are motivated as follows. As therelevance of keywords for D(.5) appeared to be quite low,we did not test lower values of λ. Similarly, we did nottest additional values of λ between .5 and 1, apart from .75,because the resulting word lists were very similar to the testedvalues. This is shown in Figure 3, where we compare thekeyword lists obtained with various values of λ above .5with keyword lists obtained with the three tested values ofλ (.5, .75, and 1), using the rank biased overlap (RBO) [56]as a similarity metric, based on the fraction of keywordsoverlapping at different ranks.

RBO is computed as follows. Let S and T be two rankedlists, and si be the keyword at rank i in S. The set of thekeywords up to rank d in S is {si : i ≤ d}, noted as S1:d.RBO is calculated as in Eq. 9, in which D is the size of theranked lists.

RBO(S, T ) =1∑D

d=1(12 )d−1

D∑d=1

(1

2)d−1|S1:d ∩ T1:d||S1:d ∪ T1:d|

(9)

The variations of RBO across the sampled values of λ, withrespect to three main configurations of the system, show thatthese values can indeed be clustered into three groups: .5 ≤λ < .7 can be clustered around λ = .5 as a representative value;.7 ≤ λ ≤ .8 can be assigned to the λ = .75 cluster; and .8< λ ≤ 1 can be represented by λ = 1. Therefore, in our humanevaluations, we will consider only the D(.5), D(.75) and D(1),and note the latter as TS.

2) Measuring Topical Diversity: First of all, we comparedthe four keyword extraction methods (WF, TS, D(.5) andD(.75)) in terms of the diversity of their results over theconcatenated fragments of the Fisher Corpus, by using α-NDCG (Eq. 6) to measure how evenly the extracted keywordswere distributed across the three topics of each fragment.Figure 4 shows results averaged over the 11 three-topic conver-sation fragments of the Fisher Corpus, for various sizes of thekeyword set, between 1 and 15. The average α-NDCG valuesfor D(.75) and D(.5) are similar, and they are clearly higherthan those for WF and TS for all ranks (except, of course,

9

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

α-N

DC

G(k

)

Number of keywords(k)

D(0.50)

D(0.75)

TS

WF

Fig. 4. Average α-NDCG over the 11 three-topic conversation fragments of the Fisher Corpus, for a number of keywords varying from 1 to 15. The mostdifficult task is to extract exactly one keyword from each topic, hence the lowest scores are for three keywords. The best performing methods are always thediversity-preserving ones, D(.5) and D(.75).

0.00.10.20.30.40.50.60.70.80.91.0

0.5 0.6 0.7 0.8 0.8 0.9 1.0

rank-biased overlap value of D(1) vs. D(λ)

rank-biased overlap value of D(.5) vs. D(λ)

rank-biased overlap value of D(.75) vs. D(λ)

λ value

rank

bia

sed

over

lap

valu

e

Fig. 3. Comparison of keyword lists generated by D(λ) when .5 ≤ λ ≤ 1using the rank biased overlap metric (RBO) computed between D(λ) and thethree keyword lists generated by D(1), D(.5) and D(.75). The differences inRBO are small enough to allow clustering around the three values of λ.

in the case of a single keyword in which they coincide). Thevalues for TS are particularly low, and only increase for a largenumber of keywords, demonstrating that TS does not cope wellwith topic diversity, but on the contrary emphasizes keywordsfrom the dominant topic. The values for WF are more uniformas it does not consider topics at all.

3) Measuring Keyword Relevance: We performed binarycomparisons between the outputs of each keyword extractionmethod, using crowdsourcing, over 11 fragments from themanual transcripts of the Fisher Corpus and 8 fragments fromthe manual transcripts of the AMI Corpus. The goal is to rankthe methods, so we only report here on the binary comparisonsthat allowed us to determine the ordering of the four methods,and exclude redundant comparisons.

Table III (in Section IV-B above) shows as an illustrationthe judgments that were collected when comparing the outputof D(.75) with TS on the 8 HITs of the AMI Corpus. Workerstended to disagree about the first two HITs, but then clearlyfound that the keywords extracted by D(.75) for the subsequentsix HITs better represented the conversation compared to TS.For these results, our consolidated relevance score (Eq. (5)) is78% for D(.75) vs. 22% for TS.

The averaged relevance values (consolidated scores) for allcomparisons needed to rank the four methods are shown inTable IV, separately for the manual transcripts of both Fisher

TABLE IV. COMPARATIVE RELEVANCE SCORES BASED ON HUMANJUDGMENTS FOR FOUR KEYWORD EXTRACTION METHODS. THE

FOLLOWING RANKING CAN BE INFERRED: D(.75) > TS > WF > D(.5).

Corpus Compared methods Relevance (%)(m1 vs. m2) m1 m2

Fisher D(.75) vs. TS 68 32manual TS vs. WF 82 18transcripts WF vs. D(.5) 95 5AMI D(.75) vs. TS 78 22manual TS vs. WF 60 40transcripts WF vs. D(.5) 78 22AMI D(.75) vs. TS 79 21ASR TS vs. WF 65 35transcripts WF vs. D(.5) 73 27

and AMI corpora. Although the exact differences vary, thehuman judgments over both corpora indicate the followingranking: D(.75) > TS > WF > D(.5). The optimal valueof λ is thus .75, and with this value, our diversity-awaremethod D(.75) extracts keyword sets that are judged to bemore representative than those extracted by TS or WF. Thedifferences between TS and WF, as well as WF and D(.5)are larger for the Fisher Corpus, likely due to the artificialfragments with three topics, but they are still visible on thenatural fragments of the AMI Corpus. The low scores of D(.5)are due to the low overall relevance of keywords. In particular,the comparative relevance of D(.75) vs. D(.5) on the FisherCorpus is very large (96% vs. 4%).

4) Measuring Noise Reduction Power: We performed binarycomparisons between the keyword lists obtained by four key-word extraction methods, using crowdsourcing, over the 8 frag-ments from the ASR transcripts of the AMI Corpus producedby the real time ASR system. The averaged relevance valuesfor all comparisons needed to rank these methods are shown atthe bottom of Table IV. The differences between comparisonvalues are similar for the ASR transcripts compared to themanual ones, although we notice a degradation of WF dueto ASR noise. As D(.75) still outperforms TS, the rankingremains unchanged in the presence of the ASR noise: D(.75)> TS > WF > D(.5).

We have counted the number of keywords selected by eachmethod among ASR errors, which were artificially generatedas explained in Section IV-A, as in this case these wordsare precisely known. The average numbers of such erroneous

10

keywords are shown in Figure 5 for a noise level varyingfrom 5% to 50% on the AMI Corpus. The results show thatD(.75) selects a smaller number of noisy keywords comparedto TS and WF. The WF method does not consider topic andonly selects words with higher frequency, so it may selectnoisy keywords if they correspond to a systematic mistake ofthe ASR system. Conversely, if noisy words are located ininsignificant topics, the probability of selection by both TSand D(.75) will be reduced because both select the keywordsplaced in the main topics. Moreover, if a systematic ASR errorgenerates words that produce a main topic, the advantage ofthe D(.75) over TS is that it selects a smaller number of noisykeywords.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5Aver

age

No.

of s

elec

ted

nois

y ke

ywor

ds

Percentage of ASR noise

TSD(.75)WF

Fig. 5. Average number of noisy keywords chosen by the algorithms overthe 8 conversation fragments of the AMI Corpus, for a varying percentage ofartificial ASR noise from 5% to 50%. The best performing method is D(.75).

B. Retrieval Relevance ScoresIn this section, we compare the retrieval results of queries

built from keyword lists by performing again binary compar-isons, using crowdsourcing to collect human judgments. Eachquery is passed to the Lucene search engine over the EnglishWikipedia articles. We consider two methods for derivingimplicit queries from keyword lists, to which we refer as singlequery and respectively multiple queries. A single query is madeby simply using the entire keyword set, while multiple queriesare constructed by dividing the keyword set into multipletopically-independent sub-sets (see Section III-B) and usingeach sub-set as one implicit query, merging afterward theresults into a unique document set (see Section III-C).

In previous versions of our system [7], we used the entireconversation fragment, minus the stopwords, as a single (long)query. We have also evaluated this approach, comparing itsresults with those of the keyword list based on word frequency(WF). We found that WF outperformed the use of the entirefragment, with 87% vs. 13% comparative relevance. Therefore,we will not discuss this method any further. Several previousjust-in-time retrieval systems such as the Remembrance Agentalso used WF rather than all words.

Firstly, we built single queries from the keyword sets pro-vided by the D(.75), TS and WF keyword extraction methods,and compared the three resulting document sets. Secondly, webuilt multiple queries from the same methods and performed

similar comparisons between the resulting document sets.Finally, we compared the best results of multiple queries withthe best results of single queries. This procedure was usedbecause evaluation with human subjects is time-consuming,therefore we attempted to carried out the minimal number ofcomparisons allowing the ordering of the methods.

1) Comparison Across Single Queries: Binary comparisonswere performed between the retrieval results from singlequeries based on D(.75), TS, and WF over 35 fragments fromthe ELEA Corpus. As explained above in Section IV-B, theworkers compared two document lists in terms of the relevanceor utility of suggested documents to the meeting participantsat the time of the corresponding fragment, represented throughits transcript.

TABLE V. COMPARATIVE RELEVANCE SCORES OF DOCUMENT RESULTLISTS USING SINGLE QUERIES OBTAINED FROM THREE KEYWORDEXTRACTION METHODS ON THE ELEA CORPUS. THE FOLLOWING

RANKING CAN BE INFERRED: D(.75) > WF > TS.

Compared methods Relevance (%)(m1 vs. m2) m1 m2

WF vs. TS 54 46D(.75) vs. WF 58 42D(.75) vs. TS 70 30

The average relevance values (for the metric described inSection IV-B) for the comparisons needed to rank the threemethods are shown in Table V. The human judgments indicatethe following ranking: D(.75) > WF > TS which shows thesuperiority of diversity-aware keyword extraction technique interms of the relevance of the resulting document sets whenthese keywords are used as a single query.

2) Comparison Across Multiple Queries: Binary compar-isons were then performed between the retrieval results ofmultiple topically-disjoint queries. Multiple queries were pre-pared from the keyword lists obtained from the TS and D(.75)keyword extraction methods. The two tested methods arenoted CTS and CD(.75) (with ‘C’ for clusters of keywords),respectively derived from the TS and D(.75) keyword lists.Clustering the results of WF is unpractical since the methoddoes not rely on topic modeling. Human judgments gatheredover the 35 fragments from the ELEA Corpus show thatCD(.75) outperforms CTS, with an averaged relevance valueof 62% vs. 38% as shown in Table VI.

TABLE VI. COMPARATIVE RELEVANCE SCORES OF DOCUMENTRESULTS USING CD(0.75), D(.75), CTS, AND TS ON THE ELEA CORPUS.METHODS USING MULTIPLE QUERIES OUTPERFORM THOSE USING SINGLE

QUERIES, AND AMONG THE FORMER, CD(0.75) SURPASSES CTS.

Compared methods Relevance (%)(m1 vs. m2) m1 m2

CD(.75) vs. D(.75) 65 35CTS vs. TS 65 35CD(.75) vs. CTS 62 38

3) Single Queries Versus Multiple Queries: Finally, wecompared single queries with multiple queries derived fromthe same keyword lists, namely D(.75) and TS, on the 35 frag-ments from the ELEA Corpus. The averaged relevance valuesobtained from human judgments, in Table VI, revealed thatusing multiple queries, for both types of keyword extraction

11

TABLE VII. EXAMPLE OF KEYWORD SETS OBTAINED BY THREE KEYWORD EXTRACTION METHODS FOR A FRAGMENT OF THE ELEA CORPUS.

WF TS D(.75)S = {whiskey, fire, axe, wool, S = {chocolate, cigarette, whiskey, S = {chocolate, cigarette, lighter,extra, lighted, light, lighter} whisk, shortening, shoe, pistol, lighter} whiskey, pistol, wool, shoe, fire}

TABLE VIII. EXAMPLE OF IMPLICIT QUERIES BUILT FROM THE KEYWORD LIST EXTRACTED FROM A FRAGMENT OF THE ELEA CORPUS. EACHIMPLICIT QUERY COVERS ONE OF THE ABSTRACT TOPICS OF THE FRAGMENT.

CTS CD(.75)q1 = {chocolate, cigarette, whiskey, q1 = {chocolate, cigarette, whiskey, lighter}whisk, shortening, lighter} q2 = {shoe, wool, lighter}q2 = {shoe, lighter} q3 = {pistol, fire, lighter}q3 = {pistol, lighter} q4 = {wool}

TABLE IX. EXAMPLE OF RETRIEVED WIKIPEDIA PAGES FROM FIVE DIFFERENT METHODS FOR A FRAGMENT OF THE ELEA CORPUS. RESULTS OFDIVERSE KEYWORD EXTRACTION (D(.75)) COVER MORE TOPICS, AND MULTIPLE IMPLICIT QUERIES REDUCE NOISE (CD(.75)).

WF TS D(.75) CTS CD(.75)Light Cigarette Wool Cigarette CigaretteLighting Lighter Cigarette Shoe WoolLight My Fire Shortening Lighter Lighter LighterLightness Shorten 25 m rapid fire pistol Shortening Mineral woolLight On Whisk Fire safe cigarettes Lighter than air ChocolateIn the Light Fly-whisk 25 m center-fire pistol Lighter (barge) Shoe

techniques, leads to more relevant document results comparedto the single queries, i.e. CD(.75) > D(.75) and CTS > TS.For both comparisons, the averaged relevance scores vary inthe same proportion, namely 65% to 35%, which also showsthat the improvement brought by multiple queries has a similarorder of magnitude for D(.75) and for TS.

4) Example of Document Results: To illustrate the superi-ority of CD(.75) over the other techniques, we consider anexample from one of the conversation fragments of the ELEACorpus, given in Appendix A. As described in Section IV, thespeakers had to select a list of 12 items vital to survive in coldmountainous conditions while they waited to be rescued. Thelists of keywords extracted for this fragment by D(.75), TS,and WF are shown in Table VII. As WF does not considertopical information, the keywords it selects (‘lighted’, ‘light’,and ‘lighter’) all revolve around the same notion.

Table VIII shows the topically-aware implicit queries pre-pared from the keyword lists provided by the D(.75) andTS methods (Table VII) ordered based on their importancein the fragment. The CTS method starts by covering thefirst main topic of this fragment with the keywords ‘choco-late’, ‘cigarette’, ‘whiskey’, ‘whisk’, and ‘shortening’. Thenit selects ‘shoe’ and ‘pistol’ to cover the second and thirdmain topics respectively. However, the CD(.75) method whichconsiders also topical diversity, first selects two keywords tocover the first main topic. Then it selects the third keywordof the first main topic only after the selection of a keywordshared by the first three main topics. Afterward, it selectedthe keywords ‘pistol’, ‘wool’, ‘shoe’, and ‘fire’ to respectivelycover the second, third and fourth main topics of the fragment.

Finally, Table IX shows the retrieval results (five highest-ranked Wikipedia pages) obtained by WF, TS, D(.75), CTS,and CD(.75). First of all, WF recommends almost no relevantdocument to participants. Queries made by the diverse keywordextraction technique (D(.75)) retrieve documents which coverthe largest number of topics mentioned in the conversation

fragment. Moreover, multiple queries (CTS and CD(.75))retrieve a large number of relevant documents compared tosingle queries (TS and D(.75)), likely because single queriesdo not separate the mixture of topics in the conversation, andlead to irrelevant results such as ‘Shorten’, ‘Whisk’, ‘Fly-whisk’ (found by TS) and ‘25 metre rapid fire pistol’, ‘Firesafe cigarettes’, ‘25 metre center-fire pistol’ (found by D(.75)).In addition, CD(.75) finds documents which cover more topicsmentioned in the conversation fragment in comparison to CTS.

VI. CONCLUSION

We have considered a particular form of just-in-time re-trieval systems which are intended for conversational environ-ments, in which they recommend to users documents that arerelevant to their information needs. We focused on modelingthe users’ information needs by deriving implicit queries fromshort conversation fragments. These queries are based onsets of keywords extracted from the conversation. We haveproposed a novel diverse keyword extraction technique whichcovers the maximal number of important topics in a fragment.Then, to reduce the noisy effect on queries of the mixture oftopics in a keyword set, we proposed a clustering technique todivide the set of keywords into smaller topically-independentsubsets constituting implicit queries.

We compared the diverse keyword extraction techniquewith existing methods, based on word frequency or topicalsimilarity, in terms of the representativeness of the keywordsand the relevance of retrieved documents. These were judgedby human raters recruited via the Amazon Mechanical Turkcrowdsourcing platform. The experiments showed that the di-verse keyword extraction method provides on average the mostrepresentative keyword sets, with the highest α-NDCG value,and leading – through multiple topically-separated implicitqueries – to the most relevant lists of recommended documents.Therefore, enforcing both relevance and diversity brings aneffective improvement to keyword extraction and document

12

retrieval. The keyword extraction method could be improvedby considering n-grams of words in addition to individualwords only, but this requires some adaptation of the entireprocessing chain.

Our current goals are to process also explicit queries, andto rank document results with the objective of maximizingthe coverage of all the information needs, while minimizingredundancy in a short list of documents. Integrating thesetechniques in a working prototype should help users to findvaluable documents immediately and effortlessly, without in-terrupting the conversation flow, thus ensuring the usability ofour system. In the future, this will be tested with human usersof the system within real-life meetings.

ACKNOWLEDGMENTS

The authors are grateful to the Swiss National ScienceFoundation (SNSF) for its financial support through the IM2NCCR on Interactive Multimodal Information Management(see www.im2.ch), as well as to the Hasler Foundation for theREMUS project (n. 13067, Re-ranking Multiple Search Re-sults for Just-in-Time Document Recommendation). We thankDairazalia Sanchez-Cortes and the Idiap Social Computinggroup for access to the ELEA Corpus. We also acknowledgethe anonymous reviewers for their precise comments andinsightful remarks that improved the quality and clarity of oursubmission.

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APPENDIX ACONVERSATION FRAGMENT FROM THE ELEA CORPUS

The following transcript of a four-party conversation (speak-ers A–D) was submitted to the document recommendationsystem. The keyword lists, queries and document retrieved arerespectively shown in Tables VII, VIII, and IX. A: that should

be almost the same okay.B: of the same proportions.A: so now we have the the axe, the extra clothing and the whiskey.C: where is the whiskeyA: whiskey is useless, I dont want if nobody is bothered if we havefire but that is -B: can whisk can whiskey be fire? can whiskey be lighted -C: err I thought about it and err -A: yes we may but yes, it may be possible yes whiskey can be goodfor the fire; but I dont know if it is highly flammable.B: well it can be it if it can be fired I mean lighted it can be thefluid of the cigarette lighter.C: yes but I think it is not strong enough.A: I think it doesntD: almost done?A: almost done. I am to put first the clothes then the axe, and thenthe whiskey.B: yes.A: because I feel like I didnt have that -B: but but I mean the last two things it doesnt it doesnt matter toomuch I think.A: okay. so I am yes we have like the canvas first, then the chocolate,shortening; the cigarette lighter, the pistol; and the newspaper. andthen the clothes, the axe, whiskey, map, compass. wool, steel wool.wool.B: where did you put the newspaper?A: to err light the fire.B: okay.A: and the to light in your shoes, to get warmer.

14

B: do we have okay. then we have extra shirts and pants. yes.D: are you done?

APPENDIX BPROOF FOR SECTION III

LEMMA: f(rS,z) defined in Eq. 3 is a monotone nonde-creasing submodular function.

PROOF: The property is illustrated in Fig. 6 for variousvalues of λ, and is proven as follows:

To show that f(rS,z) is monotone nondecreasing, let A andB be two arbitrary sets of keywords such that A ⊆ B, and letus show that f(rA,z) ≤ f(rB,z). If C = B \A, then:

f(rB,z) = f(rA∪C,z) = βz · rλA∪C,z = βz · (rA,z + rC,z)λ

If we substitute βz ·(rA,z+rC,z)λ with its binomial expansionby an infinite series (because λ is not an integer), then:

f(rB,z) = βz ·

(rλA,z +

∞∑k=1

(λk

)rλ−kA,z r

kC,z

).

Since rA,z and rC,z are obtained by summing over positive

probability values,∑∞k=1

(λk

)rλ−kA,z r

kC,z is a positive value.

So we conclude that f(rA,z) ≤ f(rB,z), and the function ismonotone.

Second we prove that f(rS,z) has the diminishing returnsproperty. Let A and B be two arbitrary sets of keywords suchthat A ⊆ B. Let w be a keyword not in B, and A′ = A ∪{w} and B′ = B ∪ {w}. We will now show that f(rB′,z) −f(rB,z) ≤ f(rA′,z)−f(rA,z), which is the diminishing returnsproperty.

f(rA′,z)− f(rA,z) = βz · rλA′,z − βz · rλA,z= βz · (rA,z + p(z|w))λ − βz · rλA,z

If we substitute βz · (rA,z + p(z|w))λ with its binomialexpansion, as above, then:

f(rA′,z)− f(rA,z) =

βz ·

(rλA,z +

∞∑k=1

(λk

)rλ−kA,z p(z|w)

k − rλA,z

)=

βz ·∞∑k=1

(λk

)rλ−kA,z p(z|w)

k.

Similarly, we can establish that

f(rB′,z)− f(rB,z) = βz ·∞∑k=1

(λk

)rλ−kB,z p(z|w)

k.

Since A ⊆ B then rλA,z ≤ rλB,z . We also knowthat (λ − k) < 0 for all positive integers k, be-cause 0 ≤ λ ≤ 1. So we have rλ−kB,z ≤ rλ−kA,z ,

and consequently βz ·∑∞k=1

(λk

)rλ−kB,z p(z|w)k ≤ βz ·

∑∞k=1

(λk

)rλ−kA,z p(z|w)k, which concludes the proof for

diminishing returns.Since it is monotone nondecreasing and has diminishing

returns, f(rS,z) is a monotone submodular function. �

0 0.5 1 1.50

0.2

0.4

0.6

0.8

The contribution of topic z for the keyword set S (rS,z

)

The

rew

ard

func

tion

for t

opic

z (f

z)

λ = 1.00λ = 0.75λ = 0.50λ = 0.25

Fig. 6. The reward value given to the topic z is decreasing when thecontribution of this topic is increasing for various λ values (1, 0.75, 0.5,and 0.25) and βz = 0.5.

Maryam Habibi received an MSc (2010) in artificialintelligence and a BSc (2008) in hardware engineer-ing from Sharif University of Technology, Tehran,Iran.

She is currently a Research Assistant at the IdiapResearch Institute (Martigny, Switzerland) and aPhD Student at the Electrical Engineering depart-ment of the Ecole Polytechnique Federale de Lau-sanne (EPFL) under the supervision of Prof. HerveBourlard and Dr. Andrei Popescu-Belis.

She has been a member of a team working on thedesign and implementation of a telephone call system for Tejarat Bank at ASRGooyeshPardaz Company (Tehran, Iran) and has worked at the communicationdepartment of IranAir Company (Tehran, Iran).

Andrei Popescu-Belis graduated from the EcolePolytechnique (Paris, France) in 1995, with majorsin mathematics and computer science. He receivedan MS degree in artificial intelligence from theUniversity of Paris VI in 1996, and a PhD degreein computer science and natural language processingfrom LIMSI-CNRS, University of Paris XI, in 1999.

He is currently a Senior Researcher at the IdiapResearch Institute (Martigny, Switzerland), a Lec-turer at the Ecole Polytechnique Federale de Lau-sanne (EPFL), and the head of Idiap’s NLP group.

He has been a Postdoc at UCSD and a Senior Research Assistant at ISSCO,University of Geneva.

Dr. Popescu-Belis has over 100 peer-reviewed publications in human lan-guage technology, information retrieval, and multimodal interactive systems,including two edited books. He has been involved in several large Swiss andinternational research projects.