Paper:
Joint Modeling of Content and Discourse Relations in Dialogues
We present a joint modeling approach to identify salient discussion points in spoken meetings as well as to label the discourse relations between speaker turns. A variation of our model is also discussed when discourse relations are treated as latent variables. Experimental results on two popular meeting corpora show that our joint model can outperform SVM-based classifiers for both phrase-based content selection and discourse relation prediction tasks. We also evaluate our model on predicting the consistency among team members’ understanding of their group decisions. Classifiers trained with features constructed from our model achieve significant better predictive performance than the state-of-the-art.
1 Introduction
Goal-oriented dialogues, such as meetings, negotiations, or customer service transcripts, play an important role in our daily life. Automatically extracting the critical points and important outcomes from dialogues would facilitate generating summaries for complicated conversations, understanding the decision-making process of meetings, or analyzing the effectiveness of collaborations.
We are interested in a specific type of dialogues — spoken meetings, which is a common way for collaboration and idea sharing. Previous work (Kirschner et al., 2012) has shown that discourse structure can be used capture the main discussion points and arguments put forward during problem-solving and decision-making processes in meetings. Indeed, content of different speaker turns do not occur in isolation, and should be interpreted within the context of discourse. Meanwhile, content can also reflect the purpose of speaker turns, thus facilitate with discourse relation understanding. Take the meeting snippet from AMI corpus (Carletta et al., 2006) in Figure 1 as
D: Three different types of batteries. Um can either use a hand dynamo, or the kinetic type ones, you know that they use in watches, or else uh a solar powered one.
B: Um the bat uh the battery for a a watch wouldn't require a lot of power, would be my one query. Is a kinetic one going to be able to supply enough power?
D: Yeah, I don't think it would. C: Yeah.
D: We should probably just use conventional batteries. B: Which I suppose as well would allow us to go off the shelf again, you'd say ?
D: Yeah.
Uncertain
Option
an example. This discussion is annotated with discourse structure based on the Twente Argumentation Schema (TAS) by Rienks et al. (2005), which focuses on argumentative discourse information. As can be seen, meeting participants evaluate different options by showing doubt (UNCERTAIN), bringing up alternative solution (OPTION), or giving feedback. The discourse information helps with the identification of the key discussion point, i.e., “which type of battery to use”, by revealing the discussion flow.
To date, most efforts to leverage discourse information to detect salient content from dialogues have focused on encoding gold-standard discourse relations as features for use in classifier training (Murray et al., 2006; Galley, 2006; McKeown et al., 2007; Bui et al., 2009). However, automatic discourse parsing in dialogues is still a challenging problem (Perret et al., 2016). Moreover, acquiring human annotation on discourse relations is a timeconsuming and expensive process, and does not scale for large datasets.
In this paper, we propose a joint modeling approach to select salient phrases reflecting key dis-
2
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
cussion points as well as label the discourse relations between speaker turns in spoken meetings. We hypothesize that leveraging the interaction between content and discourse has the potential to yield better prediction performance on both phrase-based content selection and discourse relation prediction. Specifically, we utilize argumentative discourse relations as defined in Twente Argument Schema (TAS) (Rienks et al., 2005), where discussions are organized into tree structures with discourse relations labeled between nodes (as shown in Figure 1). Algorithms for joint learning and joint inference are proposed for our model. We also present a variation of our model to treat discourse relations as latent variables when true labels are not available for learning. We envision that the extracted salient phrases by our model can be used as input to abstractive meeting summarization systems (Wang and Cardie, 2013; Mehdad et al., 2014). Combined with the predicted discourse structure, a visualization tool can be exploited to display conversation flow to support intelligent meeting assistant systems.
To the best of our knowledge, our work is the first to jointly model content and discourse relations in meetings. We test our model with two meeting corpora — the AMI corpus (Carletta et al., 2006) and the ICSI corpus (Janin et al., 2003). Experimental results show that our model yields accuracy of 63.2 and 59.2 on phrase selection and discourse prediction on AMI, which is significantly better than comparisons based on Support Vector Machines (SVM) classifiers (accuracy of 57.8 and 51.2). Our model trained with latent discourse also outperforms SVMs on both AMI and ICSI corpora. We further evaluate the usage of selected phrases as extractive meeting summaries. Results evaluated by ROUGE (Lin and Hovy, 2003) demonstrate that our system summaries obtain a ROUGE-SU4 F1 score of 21.3 on AMI corpus, which outperforms two utterancelevel extractive summarization baselines that select the longest and the most representative utterance from each discussion.
Moreover, since both content and discourse structure are critical for building shared understanding among participants (Mulder et al., 2002; Mercer, 2004), we further investigate whether our learned model can be utilized to predict the consistency among team members’ understanding of their group decisions. This task is first defined as
consistency of understanding (COU) by Kim and Shah (2016), who have labeled a portion of AMI discussions with consistency or inconsistency labels. We construct features from our model predictions to capture different discourse patterns and word entrainment scores for discussion with different COU level. Results on AMI discussions show that SVM classifiers trained with our features significantly outperform the state-of-the-art results (Kim and Shah, 2016) (F1: 63.1 vs. 50.5) and non-trivial baselines.
2 Related Work
Our model is inspired by research work that leverages discourse structure for identifying salient content in conversations, which is still largely reliant on features derived from gold-standard discourse labels (McKeown et al., 2007; Murray et al., 2010; Bokaei et al., 2016). There is much less work that jointly predicts the importance of content along with the discourse structure in dialogus. Oya and Carenini (2014) employs Dynamic Conditional Random Field to recognize sentences in email threads for use in summary as well as their dialogue acts. Only local discourse structures from adjacent utterances are considered. Our model is built on tree structures, which captures more global information.
Our work is also in line with keyphrase identification or phrase-based summarization for conversations. Due to the noisy nature of dialogues, recent work focuses on identifying summary-worthy phrases from meetings (Fernández et al., 2008; Riedhammer et al., 2010) or email threads (Loza et al., 2014). Our work also targets at detecting salient phrases from meetings, but focuses on the joint modeling of critical discussion points and discourse relations held between them.
For the area of discourse analysis in dialogues, a significant amount of work has been done in predicting local discourse structures, such as recognizing dialogue acts or social acts of adjacent utterances from phone conversations (Stolcke et al., 2000; Kalchbrenner and Blunsom, 2013; Ji et al., 2016), spoken meetings (Dielmann and Renals, 2008), or emails (Cohen et al., 2004). Although discourse information from non-adjacent turns has been studied in the context of online discussion forums (Ghosh et al., 2014) and meetings (HakkaniTur, 2009), none of them models the effect of discourse structure on content selection, which is a
3
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
gap that this work fills in.
3 The Joint Model of Content and Discourse Relations
3.1 Model Description
Our proposed model learns to jointly perform phrase-based content selection and discourse relation prediction by making use of the interaction between the two sources of information. Assume that a meeting discussion is denoted as x, where x consists of a sequence of discourse units x = {x1, x2, · · · , xn}. Each discourse unit can be a complete speaker turn or a part of it. As demonstrated in Figure 1, a tree-structured discourse diagram is constructed for each discussion with each discourse unit xi as a node of the tree. In this work, we consider the argumentative discourse structure by Twente Argument Schema (TAS) (Rienks et al., 2005). For each note xi, it is attached to another node xi′ (i′ < i) in the discussion, and a discourse relation di is hold on the link 〈xi, xi′〉 (di is empty if xi is the root). Let t denote the set of links 〈xi, xi′〉 in x. Following previous work on discourse analysis in meetings (Rienks et al., 2005; Hakkani-Tur, 2009), we assume that the attachment structure between discourse units are given during both training and testing.
A set of candidate phrases are extracted from each discourse unit xi, from which salient phrases that contain gist information will be identified. We obtain constituent and dependency parses for utterances using Stanford parser (Klein and Manning, 2003). We restrict eligible candidate to be a noun phrase (NP), verb phrase (VP), prepositional phrase (PP), or adjective phrase (ADJP) with at most 5 words, and its head word cannot be a stop word. If a candidate is a parent of another candidate in the constituent parse tree, we will only keep the parent. We further merge a verb and a candidate noun phrase into one candidate if the later is the direct object or subject of the verb. For example, from utterance “let’s use a rubber case as well as rubber buttons”, we can identify candidates “use a rubber case” and “rubber buttons”. For xi, the set of candidate phrases are denoted as ci = {ci,1, ci,2, · · · , ci,mi}, where mi is the number of candidates. ci,j takes a value of 1 if the corresponding candidate is selected as salient phrase; otherwise, ci,j is equal to 0. All candidate phrases in discussion x are represented as c.
We then define a log-linear model with feature
parameters w for the candidate phrases c and discourse relations d in x as: p(c,d|x,w) ∝ exp[w · Φ(c,d,x)]
∝ exp[w · n∑
i=1,<xi,xi′>∈t
φ(ci, di, di′ ,x)]
∝ exp[ n∑
i=1,<xi,xi′>∈t
(wc · mi∑ j=1 φc(ci,j ,x)
+ wd · φd(di, di′ ,x) + wcd · mi∑ j=1 φcd(ci,j , di,x))]
(1)
Here Φ(·) and φ(·) denote feature vectors. We utilize three types of feature functions: (1) content-only features φc(·), which capture the importance of phrases, (2) discourse-only features φd(·), which characterize the (potentially higherorder) discourse relations, and (3) joint features of content and discourse φcd(·), which model the interaction between the two. wc, wd, and wcd are corresponding feature parameters. Detailed feature descriptions can be found in Section 3.4. Discourse Relations as Latent Variables. As we mentioned in the introduction, acquiring labeled training data for discourse relations is a timeconsuming process since it would require human annotators to inspect the full discussions. Therefore, we further propose a variation of our model where it treats the discourse relations as latent variables, so that p(c|x,w) = ∑ d p(c,d|x,w). Its learning algorithm is slightly different as described in the next section.
3.2 Joint Learning for Parameter Estimation
For learning the model parameters w, we employ an algorithm based on SampleRank (Rohanimanesh et al., 2011), which is a stochastic structure learning method. In general, the learning algorithm constructs a sequence of configurations for sample labels as a Markov chain Monte Carlo (MCMC) chain based on a task-specific loss function, where stochastic gradients are distributed across the chain. This is suitable for our learning problem because we aim to optimize the prediction performance for both phrase selection and discourse relations with various types of features.
The full learning procedure is described in Algorithm 1. To start with, the feature weights w is initialized with each value randomly drawn from [−1, 1]. Multiple epochs are run through all samples. For each sample, we randomly initialize the assignment of candidate phrases labels c and discourse relations d. Then an MCMC chain is con-
4
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
structed with a series of configurations σ = (c, d): at each step, it first samples a discourse structure d based on the proposal distribution q(d′|d,x), and then samples phrase labels conditional on the new discourse relations and previous phrase labels based on q(c′|c,d′,x). Local search is used for both proposal distributions. The new configuration is accepted if it improves on the score by ω(σ′). The parameters w are updated accordingly.
For the scorer ω, we use a weighted combination of F1 scores of phrase selection (F1c) and discourse relation prediction (F1d): ω(σ) = α · F1c + (1− α) · F1d. We fix α to 0.1.
When discourse relations are treated as latent, we initialize discourse relations for each sample with a label in {1, 2, . . . ,K} if there are K relations indicated, and we only use F1c as the scorer.
Input : X = {x}: discussions in the training set, η: learning rate, : number of epochs, δ: number of sampling rounds, ω(·): scoring function, Φ(·): feature functions
Output: feature weights 1|W| ∑ w∈W w Initialize w; W ← {w}; for e = 1 to do
for x in X do // Initialize configuration for
x Initialize c and d; σ = (c,d); for s = 1 to δ do
// New configuration via local search d′ ∼ qd(·|x,d); c′ ∼ qd(·|x, c,d′); σ′ = (c′,d′); σ+ = arg maxσ̃∈{σ,σ′} ω(σ̃); σ− = arg minσ̃∈{σ,σ′} ω(σ̃); ∇̂ = Φ(σ+)− Φ(σ−); ∆ω = ω(σ+)− ω(σ−); // Update parameters if w · ∇̂ < ∆ω & ∆ω 6= 0 then
w′ = w + η · ∇̂; Add w′ inW;
end // Accept or reject new
configuration if σ+ == σ′ then
σ = σ′ end
end end
end Algorithm 1: SampleRank-based joint learning.
3.3 Joint Inference for Prediction
Given a new sample x and learned parameters w, we predict phrase labels and discourse relations as arg maxc,d p(c,d|x,w).
Dynamic programming can be employed to carry out joint inference, however, it would
be time-consuming since our objective function has a large search space for both content and discourse labels. Hence we propose an alternating optimizing algorithm to search for c and d iteratively. Concretely, for each iteration, we first optimize on d by maximizing ∑n i=1,<xi,x′i>∈t
(wd · φd(di, di′ ,x) + wcd ·∑mi j=1 φcd(ci,j , di,x)). Message-passing (Smith and Eisner, 2008) is used to find the best d. In the second step, we search for c that max-
imizes ∑n
i=1,<xi,x′i>∈t (wc · ∑mi j=1 φc(ci,j ,x) +
wcd · ∑mi
j=1 φcd(ci,j , di,x)). We believe that candidate phrases based on the same concepts should have the same predicted label. Therefore, candidates of the same phrase type and sharing the same head word are grouped into one cluster. We then cast our task as an integer linear programming problem.1 We optimize our objective function under constraints: (1) ci,j = ci′,j′ if ci,j and ci′,j′ are in the same cluster, and (2) ci,j ∈ 0, 1, ∀i, j.
The inference process is the same for models trained with latent discourse relations.
3.4 Features
We use features that characterize content, discourse relations, and the combination of both. Content Features. For modeling the salience of content, we calculate the minimum, maximum, and average of TF-IDF scores of words and number of content words in each phrase based on the intuition that important phrases tend to have more content words with high TF-IDF scores (Fernández et al., 2008). We also consider whether the head word of the phrase has been mentioned in preceding turn, which implies the focus of a discussion. The size of the cluster this phrase belongs to is also included. Number of POS tags and phrase types are counted to characterize the syntactic structure. Previous work (Wang and Cardie, 2012) has found that a discussion usually ends with decision-relevant information. We thus identify the absolute and relative positions of the turn containing the candidate phrase in the discussion. Finally, we record whether the candidate phrase is uttered by the main speaker, who speakers the most words in the discussion. Discourse Features. For each discourse unit, we
1We use lpsolve: http://lpsolve. sourceforge.net/5.5/.
5
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
collect the dialogue act types of the current unit and its parent node in discourse tree, whether there is any adjacency pair held between the two nodes (Hakkani-Tur, 2009), and the Jaccard similarity between them. We record whether two turns are uttered by the same speaker, for example, ELABORATION is commonly observed between the turns from the same participant. We also calculate the number of candidate phrases based on the observation that OPTION and SPECIALIZATION tend to contain more informative words than POSITIVE feedback. Length of the discourse unit is also relevant. Therefore, we compute the time span and number of words. To incorporate global structure features, we encode the depth of the node in the discourse tree and the number of its siblings. Finally, we include an order-2 discourse relation feature that encodes the relation between current discourse unit and its parent, and the relation between the parent and its grandparent if it exists. Joint Features. For modeling the interaction between content and discourse, the discourse relation is added to each content feature to compose a joint feature. For example, if candidate c in discussion x has a content feature φ[avg−TFIDF ](c,x) with a value of 0.5, and its discourse relation d is POSITIVE, then the joint feature takes the form of φ[avg−TFIDF,Positive](c, d,x) = 0.5.
4 Datasets and Experimental Setup
Meeting Corpora. We evaluate our joint model on two meeting corpora with rich annotations: the AMI meeting corpus (Carletta et al., 2006) and the ICSI meeting corpus (Janin et al., 2003). AMI corpus consists of 139 scenario-driven meetings, and ICSI corpus contains 75 naturally occurring meetings. Both of the corpora are annotated with dialogue acts, adjacency pairs, and topic segmentation. We treat each topic segment as one discussion, and remove discussions with less than 10 turns or labeled as “opening”. 694 discussions from AMI and 1139 discussions from ICSI are extracted, and these two datasets are henceforth referred as AMI-FULL and ICSI-FULL. Acquiring Gold-Standard Labels. Both corpora contain human constructed abstractive summaries and extractive summaries on meeting level. Short abstracts, usually in one sentence, are constructed by meeting participants — participant summaries,
and external annotators — abstractive summaries. Dialogue acts that contribute to important output of the meeting, e.g. decisions, are identified and used as extractive summaries, and some of them are also linked to the corresponding abstracts.
Since the corpora do not contain phrase-level importance annotation, we induce gold-standard labels for candidate phrases based on the following rule. A candidate phrase is considered as a positive sample if its head word is contained in any abstractive summary or participant summary.
Furthermore, a subset of discussions in AMIFULL are annotated with discourse structure and relations based on Twente Argumentation Schema (TAS) by Rienks et al. (2005)2. A tree-structured argument diagram (as shown in Figure 1) is created for each discussion or a part of the discussion. The nodes of the tree contain partial or complete speaker turns, and discourse relation types are labeled on the links between the nodes. In total, we have 129 discussions annotated with discourse labels. This dataset is called AMI-SUB hereafter. Experimental Setup. 5-fold cross validation is used for all experiments. All real-valued features are uniformly normalized to [0,1]. For the joint learning algorithm, we use 10 epochs and carry out 50 sampling for MCMC for each training sample. The learning rate is set to 0.01. We run the learning algorithm for 20 times, and use the average of the learned weights as the final parameter values. For models trained with latent discourse relations, we fix the number of relations to 9. Baselines and Comparisons. For both phrasebased content selection and discourse relation prediction tasks, we consider (1) a baseline that always predicts the majority label (Majority), and (2) a random baseline. Previous work has shown that Support Vector Machines (SVMs)-based classifiers achieve state-of-the-art performance for keyphrase selection in meetings (Fernández et al., 2008; Wang and Cardie, 2013) and discourse parsing for formal text (Hernault et al., 2010). Therefore, we compare with linear SVM-based classifiers, trained with the same feature set of content features or discourse features. We fix the trade-off parameter to 1.0 for all SVM-based experiments. For discourse relation prediction, we use one-vsrest strategy to build multiple binary classifiers.
2There are 9 types of relations in TAS: POSITIVE, NEGATIVE, UNCERTAIN, REQUEST, SPECIALIZATION, ELABORATION, OPTION, OPTION EXCLUSION, and SUBJECT-TO.
6
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
Acc F1 Comparisons Baseline (Majority) 60.1 37.5 Baseline (Random) 50.0 33.1 SVM (w content features in § 3.4) 57.8 54.6 Our Models Joint-Learn + Joint-Inference 63.2∗ 62.6∗ Joint-Learn + Separate-Inference 57.9 57.8 Separate-Learn 53.4 52.6 Our Models (Latent Discourse) w/ True Attachment Structure Joint-Learn + Joint-Inference 60.3∗ 60.3∗ Joint-Learn + Separate-Inference 56.4 56.2 w/o True Attachment Structure Joint-Learn + Joint-Inference 56.4 56.4 Joint-Learn + Separate-Inference 52.7 52.3
Table 1: Phrase-based content selection performance on AMI-SUB with accuracy (acc) and F1. We display results of our models trained with gold-standard discourse relation labels and with latent discourse relations. For the later, we also show results based on True Attachment Structure, where the gold-standard attachments are known, and without the True Attachment Structure. Our models that significantly outperform SVM-based model are highlighted with ∗ (p < 0.05, paired t-test). Best result for each column is in bold.
Acc F1 Comparisons Baseline (Majority) 51.2 7.5 Baseline (Random) 11.1 1.9 SVM (w discourse features in § 3.4) 51.2 22.8 Our Models Joint-Learn + Joint-Inference 58.0∗ 21.7 Joint-Learn + Separate-Inference 59.2∗ 23.4 Separate-Learn 58.2∗ 25.1
Table 2: Discourse relation prediction performance on AMI-SUB. Our models that significantly outperform SVM-based model are highlighted with ∗ (p < 0.05, paired t-test).
5 Experimental Results
5.1 Phrase Selection and Discourse Labeling
Here we present the experimental results on phrase-based content selection and discourse relation prediction. We experiment with two variations of our joint model: one is trained on goldstandard discourse relations, the other is trained by treating discourse relations as latent models as described in Section 3.1. Remember that we have gold-standard argument diagrams on the AMISUB dataset, we can thus conduct experiments by assuming the True Attachment Structure is given for latent versions. When argument diagrams are not available, we build a tree among the turns in each discussion as follows. Two turns are attached if there is any adjacency pair between them. If one turn is attached to more than one previous turns, the closest one is considered. For the rest of the turns, they are attached to the preceding turn.
We also investigate whether joint learning and joint inference can produce better prediction performance. We consider joint learning with separate inference, where only content features or discourse features are used for prediction. We further study learning separate classifiers for content selection and discourse relations without joint features (Separate-Learn).
We first show the phrase selection and discourse relation prediction results on AMI-SUB in Tables 1 and 2. As shown in Table 1, our models, trained with gold-standard discourse relations or latent ones with true attachment structure, yield significant better accuracy and F1 scores than SVM-based classifiers trained with the same feature sets for phrase selection (paired t-test, p < 0.05). Moreover, both Tables 1 and 2 demonstrate that joint learning usually produces superior performance for both tasks than separate learning. Combined with joint inference, our model obtains the best accuracy and F1 on phrase selection. This indicates that leveraging the interplay between content and discourse boost the prediction performance. Similar results are achieved on AMI-FULL and ICSI-FULL in Table 3, where latent discourse relations without true attachment structure are employed for training.
5.2 Phrase-Based Extractive Summarization
We further evaluate whether the prediction of the content selection component can be used for summarizing the key points on discussion level. For each discussion, salient phrases identified by our model are concatenated in sequence for use as the summary. We consider two types of gold-standard summaries. One is utterance-level extractive summary, which consists of human labeled summaryworthy utterances. The other is abstractive summary, where we collect human abstract with at least one link from summary-worthy utterances.
7
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
Extractive Summaries as Gold-Standard ROUGE-1 ROUGE-SU4 Len Prec Rec F1 Prec Rec F1 Longest DA 30.9 64.4 15.0 23.1 58.6 9.3 15.3 Centroid DA 17.5 73.9 13.4 20.8 62.5 6.9 11.3 SVM 49.8 47.1 24.1 27.5 22.7 10.7 11.8 Our Model 66.6 45.4 44.7 41.1∗ 24.1∗ 23.4∗ 20.9∗ Our Model-latent 85.9 42.9 49.3 42.4∗ 21.6 25.7∗ 21.3∗ Abstractive Summaries as Gold-Standard
ROUGE1 ROUGE-SU4 Len Prec Rec F1 Prec Rec F1
Longest DA 30.9 14.8 5.5 7.4 4.8 1.4 1.9 Centroid DA 17.5 24.9 5.6 8.5 11.6 1.4 2.2 SVM 49.8 13.3 9.7 9.5 4.4 2.4 2.4 Our Model 66.6 12.6 18.9 13.1∗ 3.8 5.5∗ 3.7∗ Our Model-latent 85.9 11.4 20.0 12.4∗ 3.3 6.1∗ 3.5∗
Table 4: ROUGE scores for phrase-based extractive summarization evaluated against human-constructed utterance-level extractive summaries and abstractive summaries. Our models that statistically significantly outperform all comparisons are highlighted with ∗ (p < 0.05, paired t-test).
We calculate scores based on ROUGE (Lin and Hovy, 2003), which is a popular tool for evaluating text summarization (Gillick et al., 2009; Liu and Liu, 2010). ROUGE-1 (unigrams) and ROUGE-SU4 (skip-bigrams with at most 4 words in between) are used. Following previous work on meeting summarization (Riedhammer et al., 2010; Wang and Cardie, 2013), we consider two dialogue act-level summarization baselines: (1) LONGEST DA in each discussion is selected as the summary, and (2) CENTROID DA, the one with the highest TF-IDF similarity with all DAs in the discussion. We also compare with summaries consisting of salient phrases predicted by an SVM classifier trained with on our content features.
From the results in Table 4, we can see that phrase-based extractive summarization methods by our system and SVM-based classifiers can yield better ROUGE scores for recall and F1 than baselines that extract the whole sentences. Meanwhile, our system significantly outperforms the SVMbased classifiers when evaluated on ROUGE recall and F1, while achieving comparable precision.
Sample summaries by our model along with two baselines are displayed in Figure 2. Utterancelevel extract-based baselines unavoidably contain disfluency and unnecessary details. Our phrasebased extractive summary is able to capture the key points from both the argumentation process and important outcomes of the conversation.
5.3 Further Analysis and Discussions
Features Analysis. We first discuss salient features with top weights learned by our joint model.
For content features, main speaker tends to utter more salient content. Higher TF-IDF scores also indicate important phrases. For discourse features, structure features matter the most. For instance, jointly modeling the discourse relation of the parent node along with the current node can lead to better inference. An example is that giving more details on the proposal (ELABORATION) tends to lead to POSITIVE feedback. For joint features, features that composite “phrase mentioned in previous turn” and relation POSITIVE feedback or REQUEST yield higher weight, which are indicators for both key phrases and discourse relations. We also find that main speaker information composite with ELABORATION and UNCERTAIN are associated with high weights. Error Analysis and Potential Directions. Taking a closer look at our prediction results, one major source of incorrect prediction for phrase selection is based on the fact that similar concepts might be expressed in different ways, and our model predicts inconsistently for different variations. For example, participants use both “thick” and “two centimeters” to talk about the desired shape of a remote control. However, our model does not group them into the same cluster and later makes different predictions. Furthermore, identifying discourse relations in dialogues is still a challenging task. For instance, “I wouldn’t choose a plastic case” should be labeled as OPTION EX-
8
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
CLUSION, if the previous turns talk about different options. Otherwise, it can be labeled as NEGATIVE. Therefore, models that better handle semantics and context need to be considered.
6 Predicting Consistency of Understanding
In this section, we test whether our joint model can be utilized to predict the consistency among team members’ understanding of their group decisions, which is defined as consistency of understanding (COU) in Kim and Shah (2016).
Kim and Shah (2016) establish gold-standard COU labels on a portion of AMI discussions, by comparing participant summaries to determine whether participants report the same decisions. If all decision points are consistent, the associated topic discussion is labeled as consistent; otherwise, the discussion is identified as inconsistent. Their annotation covers the AMI-SUB dataset. Therefore, we run the prediction experiments on AMI-SUB by using the same annotation.3 Out of total 129 discussions in AMI-SUB, 86 discussions are labeled as consistent and 43 are inconsistent.
We construct three types of features by using our model’s predicted labels. Firstly, we learn two versions of our model based on the “consistent” discussions and the “inconsistent” ones in the training set, with learned parameters wcon and wincon. For a discussion in the test set, these two models output two probabilities pcon = maxc,d P (c,d|x,wcon) and pincon = maxc,d P (c,d|x,wincon). We use pcon − pincon as a feature.
Furthermore, we consider discourse relations of length one and two from the discourse structure tree. Intuitively, some discourse relations, e.g., ELABORATION followed by several POSITIVE implied consistent understanding.
The third feature is based on word entrainment, which has been shown to correlate with task success for groups (Nenkova et al., 2008). Using the formula in Nenkova et al. (2008), we compute the average word entrainment between the main speaker who utters the most words and all the other participants. The content words in the salient phrases predicted by our model is considered for entrainment computation. Results. Leave-one-out is used for experiments.
3We thank Joseph Kim for providing the dataset and rerunning their system based on our setup.
For training, our features are constructed from gold-standard phrase and discourse labels. Predicted labels by our model is used for constructing features during testing. SVM-based classifier is used. A majority class baseline and a random baseline are constructed as well. We also consider an SVM classifier trained with ngram features (unigrams and bigrams). Finally, we compare with the state-of-the-art method in Kim and Shah (2016), where discourse-relevant features and head gesture features are utilized in Hidden Markov Models to predict the consistency label.
The results are displayed in Table 5. All SVMs trained with our features surpass the ngrams-based baseline. Especially, the discourse features, word entrainment feature, and the combination of the three, all significantly outperform the state-of-theart system by Kim and Shah (2016).
7 Conclusion
We presented a joint model for performing phraselevel content selection and discourse relation prediction in spoken meetings. Experimental results on AMI and ICSI meeting corpora showed that our model can outperform SVM-based classifiers trained with the same feature sets. Further evaluation on the task of predicting consistencyof-understanding in meetings demonstrated that classifiers trained with features constructed from our model’s predictions produced superior performance compared to state-of-the-art model.
9
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899

Review:
This paper proposes a joint model of salient phrase selection and discourse relation prediction in spoken meeting. Experiments using meeting corpora show that the proposed model has higher performance than the SVM-based classifier.  - Strengths: The paper is written to be easy to read. Technical details are described fully, and high performance is also shown in experimental evaluation. It also shows useful comparisons with related research in the field of discourse structure analysis and key phrase identification. It is interesting to note that not only the performance evaluation of phrase selection from discourse, discourse relation labeling, and summary generation as their applications, but also application to the prediction of the consistency of  understanding by team members is also verified .  - Weaknesses: Jointly Modeling salient phrase extraction and discourse relationship labeling between speaker turns has been proposed. If intuitive explanation about their interactivity and the usefulness of considering it is fully presented.  - General Discussion: SVM-based classifier is set as a comparative method in the experiment. It would be useful to mention the validity of the setting.