acl acl2010 acl2010-8 knowledge-graph by maker-knowledge-mining

8 acl-2010-A Hybrid Hierarchical Model for Multi-Document Summarization


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Author: Asli Celikyilmaz ; Dilek Hakkani-Tur

Abstract: Scoring sentences in documents given abstract summaries created by humans is important in extractive multi-document summarization. In this paper, we formulate extractive summarization as a two step learning problem building a generative model for pattern discovery and a regression model for inference. We calculate scores for sentences in document clusters based on their latent characteristics using a hierarchical topic model. Then, using these scores, we train a regression model based on the lexical and structural characteristics of the sentences, and use the model to score sentences of new documents to form a summary. Our system advances current state-of-the-art improving ROUGE scores by ∼7%. Generated summaries are less rbeydu ∼n7d%an.t a Gnedn more dc sohuemremnatr bieasse adre upon manual quality evaluations.

Reference: text


Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 edu i Abstract Scoring sentences in documents given abstract summaries created by humans is important in extractive multi-document summarization. [sent-3, score-0.326]

2 In this paper, we formulate extractive summarization as a two step learning problem building a generative model for pattern discovery and a regression model for inference. [sent-4, score-0.338]

3 We calculate scores for sentences in document clusters based on their latent characteristics using a hierarchical topic model. [sent-5, score-0.693]

4 Then, using these scores, we train a regression model based on the lexical and structural characteristics of the sentences, and use the model to score sentences of new documents to form a summary. [sent-6, score-0.335]

5 1 Introduction Extractive approach to multi-document summarization (MDS) produces a summary by selecting sentences from original documents. [sent-10, score-0.519]

6 Document Understanding Conferences (DUC), now TAC, fosters the effort on building MDS systems, which take document clusters (documents on a same topic) and description of the desired summary focus as input and output a word length limited summary. [sent-11, score-0.465]

7 Human summaries are provided for training summarization models and measuring the performance of machine generated summaries. [sent-12, score-0.299]

8 Extractive summarization methods can be classified into two groups: supervised methods that rely on provided document-summary pairs, and unsupervised methods based upon properties derived from document clusters. [sent-13, score-0.284]

9 Each candidate sentence is classified as summary or non-summary based on the features that they pose and those with highest scores are selected. [sent-21, score-0.438]

10 Such models can yield comparable or better performance on DUC and other evaluations, since representing documents as topic distributions rather than bags of words diminishes the effect of lexical variability. [sent-28, score-0.288]

11 In this paper, we present a novel approach that formulates MDS as a prediction problem based on a two-step hybrid model: a generative model for hierarchical topic discovery and a regression model for inference. [sent-30, score-0.516]

12 We investigate if a hierarchical model can be adopted to discover salient characteristics of sentences organized into hierarchies utilizing human generated summary text. [sent-31, score-0.607]

13 We present a probabilistic topic model on sentence level building on hierarchical Latent Dirichlet Allocation (hLDA) (Blei et al. [sent-32, score-0.367]

14 We construct a hybrid learning algorithm by extracting salient features to characterize summary sentences, and implement a regression model for inference (Fig. [sent-35, score-0.504]

15 We show in § 6 that our hybrid summarizer achWieeve ssh comparable (if tno otu better) iRdO sUumGmE score on the challenging task of extracting the summaries of multiple newswire documents. [sent-43, score-0.296]

16 While, earlier work on summarization depend on a word score function, which is used to measure sentence rank scores based on (semi-)supervised learning methods, recent trend of purely data-driven methods, (Barzilay and Lee, 2004; Daum e´III and Marcu, 2006; Tang et al. [sent-48, score-0.319]

17 Our objective is to discover from document clusters, the latent topics that are organized into hierarchies following (Haghighi and Vanderwende, 2009). [sent-51, score-0.348]

18 It follows that summary topics are commonly shared by many documents, while specific topics are more likely to be mentioned in rather a small subset of documents. [sent-57, score-0.53]

19 Feature based learning approaches to summarization methods discover salient features by measuring similarity between candidate sentences and summary sentences (Nenkova and Vanderwende, 2005; Conroy et al. [sent-58, score-0.829]

20 Recent studies focused on the discovery of latent topics of document sets in extracting summaries. [sent-62, score-0.307]

21 One of the challenges of using a previously trained topic model is that the new document might have a totally new vocabulary or may include many other specific topics, which may or may not exist in the trained model. [sent-64, score-0.311]

22 An alternative yet feasible solution, presented in this work, is building a model that can summarize new document clusters using characteristics of topic distributions of training documents. [sent-66, score-0.471]

23 Our approach differs from the early work, in that, we combine a generative hierarchical model and regression model to score sentences in new documents, eliminating the need for building a generative model for new document clusters. [sent-67, score-0.556]

24 3 Summary-Focused Hierarchical Model Our MDS system, hybrid hierarchical summarizer, HybHSum, is based on an hybrid learning approach to extract sentences for generating summary. [sent-68, score-0.418]

25 We discover hidden topic distributions of sentences in a given document cluster along with provided summary sentences based on hLDA described in (Blei et al. [sent-69, score-0.935]

26 We build a summary-focused hierarchical probabilistic topic model, sumHLDA, for each document cluster at sentence level, because it enables capturing expected topic distributions in given sentences di- rectly from the model. [sent-71, score-0.853]

27 Besides, document clusters contain a relatively small number of documents, which may limit the variability of topics if they are evaluated on the document level. [sent-72, score-0.477]

28 Let a given document cluster D be represented with sentences O={om}|mO=|1 and its corresponding human summary be represented with sentences S={sn? [sent-75, score-0.659]

29 816 Summary hLDA (sumHLDA): The hLDA represents distribution of topics in sentences by organizing topics into a tree of a fixed depth L (Fig. [sent-89, score-0.425]

30 Each candidate sentence om is assigned to a path com in the tree and each word wi in a given sentence is assigned to a hidden topic zom at a level lof com . [sent-92, score-1.482]

31 The sampler method alternates between choosing a new path for each sentence through the tree and assigning each word in each sentence to a topic along that path. [sent-94, score-0.476]

32 The assignments of sentences to paths are sampled sequentially: The first sentence takes the initial L-level path, starting with a single branch tree. [sent-99, score-0.313]

33 The idea is to represent each path shared by similar candidate sentences with representative summary sentence(s). [sent-105, score-0.57]

34 •A itf aea ccahn node, we nlteet summary sentences sample a path by choosing only from the existing children of that node with a probability proportional to the number of other sentences assigned to that child. [sent-107, score-0.689]

35 By choosing γs ≪ γo we suppress the generation of new branches for summary sentences and modify the γ of nCRP prior in Eq. [sent-110, score-0.438]

36 (2) For each sentence d ∈ {O ∪ S}, (a) irf e da ∈ Ose,n dternawce a path cd v nCRP(γo), eifls de i∈f dO ∈ Sra,w wdr aaw pa a path cd v nCRP(γs). [sent-114, score-0.351]

37 (c) For each word n, choose:∼ (i) lierv(eαl zd,n |θd and (ii) word wd,n | {zd,n, cd, β} Given sentence d, θd |is{ a vecto,rβ o}f topic proportions from L dimensional Dirichlet parameterized by α (distribution over levels in the tree. [sent-116, score-0.32]

38 The aim is to obtain the following samples from the posterior of: (i) the latent tree T, (ii) the level assignment z for all words, (iii) the path assignments c for all sentences conditioned on the observed words w. [sent-123, score-0.41]

39 4 Tree-Based Sentence Scoring The sumHLDA constructs a hierarchical tree structure of candidate sentences (per document cluster) by positioning summary sentences on the tree. [sent-129, score-0.825]

40 Each sentence is represented by a path in the tree, and each path can be shared by many sentences. [sent-130, score-0.323]

41 Moreover, if a path includes a summary sentence, then candidate sentences on that path are more likely to be selected for summary text. [sent-132, score-0.959]

42 In particular, the similarity of a candidate sentence om to a summary sentence sn sharing the same path is a measure of strength, indicating how likely om is to be included in the generated summary (Algorithm 1): Let com be the path for a given om. [sent-133, score-2.232]

43 We find summary sentences that share the same path with om via: M = {sn ∈ S|csn = com }. [sent-134, score-1.055]

44 The score of eachv isae:nt Menc =e i s{ csalc∈u lSat|ecd by similarity teo stchoer bee ostf matching summary sentence in M: score(om) = maxsn∈M sim(om, sn) (4) If M=ø, then score(om)=ø. [sent-135, score-0.415]

45 The efficiency of our similarity measure in identifying the best matching summary sentence, is tied to how expressive the extracted topics of our sumHLDA models are. [sent-136, score-0.48]

46 Given path com , we calculate the similarity of om to each sn, n=1. [sent-137, score-0.739]

47 sparse unigram distributions (sim1) at each topic lon com : similarity between p(wom,l |zom = l, com , vl) and p(wsn,l |zsn = l, com , vl) ? [sent-140, score-0.686]

48 distributions of topic proportions (sim2); similarity between p(zom |com ) and p(zsn |com ). [sent-142, score-0.372]

49 sim1: We define two sparse (discrete) un− igram dmistributions for candidate om and summary sn at each node l on a vocabulary identified with words generated by th? [sent-143, score-0.975]

50 at are generated from topic zom at level l on path com . [sent-153, score-0.615]

51 The discrete unigram distribution poml = p(wom,l |zom = l, com , vl) represents the probability over zall words vl assigned to topic zom at level l, by sampling only for words in wom,l. [sent-154, score-0.595]

52 Algorithm 1Tree-Based Sentence Scoring 1: Given tree T from sumHLDA, candidate and summary sentences: O = {o1, . [sent-160, score-0.372]

53 ,ee|O OT| danod summary sentences 4: on path com : M = {sn ∈ S|csn = com } 5: for summary Msen =tenc {ess n∈ ← S| 1c, . [sent-171, score-0.995]

54 (6) at each level of com by: sim1(om, sn) = L1 PlL=1 Wcom,l(pom,l,psn,l) ∗ l (7) The similarity betweenpom,l andpsn,l at each level is weighted proportional to the level lbecause the similarity between sentences should be rewarded if there is a specific word overlap at child nodes. [sent-187, score-0.495]

55 −sim2: We introduce another measure based on sentence-topic mixing proportions to calculate the concept-based similarities between om and sn. [sent-188, score-0.589]

56 We calculate the topic proportions of om and sn, represented by pzom = p(zom |com ) and pzsn = p(zsn |com ) via Eq. [sent-189, score-0.794]

57 Distribution of words in given two sentences, a candidate (om) and a summary (sn) using sub-vocabulary of words at each topic vzl . [sent-195, score-0.503]

58 Discrete distributions on the left are topic mixtures for each sentence, pzom and pzsn . [sent-196, score-0.337]

59 (6) by: sim2 (om, sn) = 10−IRcom (pzom,pzsn) (8) sim1 provides information about the similarity between two sentences, om and sn based on topicword distributions. [sent-198, score-0.633]

60 They jointly effect the sentence score and are combined in one measure: sim(om, sn) = sim1 (om, sn) ∗ sim2 (om, sn) (9) The final score for a given om is calculated from Eq. [sent-200, score-0.57]

61 b depicts a sample path illustrating sparse unigram distributions of om and sm at each level as well as their topic proportions, pzom , and pzsn . [sent-204, score-1.009]

62 In experiment 3, we discuss the effect of our tree-based scoring on summarization performance in comparison to a classical scoring method presented as our baseline model. [sent-205, score-0.295]

63 Thus, we create ngram meta-features to represent sentences instead of word n-gram frequencies: (I) nGram Meta-Features (NMF): For each document cluster D, we identify most frequent (non-stop word) unigrams, i. [sent-214, score-0.287]

64 We measure observed unigram probabilities for each wi ∈ vfreq with pD (wi) = nD(wi)/Pj|V= |1 nD(wj), where nD(wi) is the number oPf times wi appears in D and |V | is the total numPber of unigrams. [sent-217, score-0.327]

65 (II) Document Word Frequency MetaFeatures (DMF): The characteristics of sentences at the document level can be important in summary generation. [sent-221, score-0.576]

66 DMF identify whether a word in a given sentence is specific to the document in consideration or it is commonly used in the document cluster. [sent-222, score-0.339]

67 This is important because summary sentences usually contain abstract terms rather than specific terms. [sent-223, score-0.372]

68 Given sentence om, let d be the docum∈ent v that om belongs to, i. [sent-227, score-0.502]

69 We measure unigram probabilities for each wi by p(wi ∈ om) = nd(wi ∈ om)/nD (wi), where nd(wi ∈ om) is the numbe∈r of times wi appears in d and nD (wi) is the number of times wi appears in D. [sent-230, score-0.403]

70 ristics with a regression model using sentences in different document clusters. [sent-255, score-0.354]

71 Redundancy Elimination: To eliminate redundant sentences in the generated summary, we incrementally add onto the summary the highest ranked sentence om and check if om significantly repeats the information already included in the summary until the algorithm reaches word count limit. [sent-257, score-1.61]

72 A om is discarded if its similarity to any of the previously selected sentences is greater than a threshold identified by a greedy search on the training dataset. [sent-259, score-0.605]

73 6 Experiments and Discussions In this section we describe a number of experiments using our hybrid model on 100 document clusters each containing 25 news articles from DUC2005-2006 tasks. [sent-260, score-0.312]

74 Contrary to typical hLDA models, to efficiently represent sentences in summarization task, we set ascending values for Dirichlet hyper-parameter η as the level increases, encouraging mid to low level distributions to generate as many words as in higher levels, e. [sent-268, score-0.392]

75 R toOU coGmEis used for performance measure (Lin and Hovy, 2003; Lin, 2004), which evaluates summaries based on the maxium number of overlapping units between generated summary text and a set of human summaries. [sent-289, score-0.441]

76 Here, we illustrate that this prior is practical in learning hierarchical topics for summarization task. [sent-292, score-0.41]

77 We use sentences from the human generated summaries during the discovery of hierarchical topics of sentences in document clusters. [sent-293, score-0.74]

78 Since summary sentences generally contain abstract words, they are indicative of sentences in documents and should produce minimal amount of new sis: In sumHLDA topics (if not none). [sent-294, score-0.672]

79 To implement this, in nCRP prior of sumHLDA, we use dual hyper-parameters and choose a very small value for summary sentences, γs = 10e−4 ? [sent-295, score-0.296]

80 To analyze this prior, we generate a corpus of v1300 sentences of a document cluster in DUC2005. [sent-299, score-0.287]

81 1 8351831508 sumHhL DDAA33 5 8 2417 126 72 16570191 250272 34509808 870 5105 Table 1: Average # of topics per document cluster from sumHLDA and hLDA for different γ and γo and tree depths. [sent-303, score-0.353]

82 Less number of topics(nodes) in sumHLDA suggests that summary sentences share pre-existing paths and no new paths or nodes are sampled for them. [sent-308, score-0.497]

83 The score of a candidate sentence is the cosine similarity to the maximum matching summary sentence. [sent-315, score-0.484]

84 As presented in § 5, NMF is the bundle of frequency sbeansetded m inet §a- 5fe,a NtuMresF on hdoec buumnednlet c olfus frteerlevel, DMF is a bundle of frequency based metafeatures on individual document level and OF represents sentence term frequency, location, and size features. [sent-318, score-0.418]

85 HIERSUM : (Haghighi and Vanderwende, 2009) A generative summarization method based on topic models, which uses sentences as an additional level. [sent-328, score-0.471]

86 Using an approximation for inference, sentences are greedily added to a summary so long as they decrease KL-divergence. [sent-329, score-0.372]

87 We use the following HybFSum (Hybrid Flat Summarizer): To investigate the performance of hierarchical topic model, we build another hybrid model using flat LDA (Blei et al. [sent-330, score-0.373]

88 In LDA each sentence is a superposition of all K topics with sentence specific weights, there is no hierarchical relation between topics. [sent-332, score-0.357]

89 Instead of the new tree-based sentence scoring (§ 4), we present a tsriemei-lbaars emdet sheondte using topics §fro 4m), L wDeA p on sentence level. [sent-335, score-0.339]

90 Note that in LDA the topic-word distributions φ are over entire vocabulary, and topic mixing proportions for sentences θ are over all the topics discovered from sentences in a document cluster. [sent-336, score-0.854]

91 2) and topic mixing weights θ in sentences (in place of topic proportions in Eq. [sent-338, score-0.583]

92 Compared to the HybF Sum built on LDA, both HybHSum1&2 yield better performance indicating the effectiveness of using hierarchical topic model in summarization task. [sent-351, score-0.413]

93 2, due to the new hierarchical tree-based sentence scoring which characterizes sentences on deeper level. [sent-353, score-0.343]

94 This indicates that with our regression model built on training data, summaries can be efficiently generated for test documents (suitable for online systems). [sent-358, score-0.31]

95 Human annotators are given two sets of summary text for each document set, generated from two approaches: best hierarchical hybrid HybHSum2 and flat hybrid HybFSum models, and are asked to mark the better summary ( a ) Ref. [sent-360, score-1.002]

96 gk$el1uotwasifnrcdgvy9ehowt7, Figure 2: Example summary text generated by systems compared in Experiment 3. [sent-365, score-0.299]

97 according to five criteria: non-redundancy (which summary is less redundant), coherence (which summary is more coherent), focus and readability (content and not include unnecessary details), responsiveness and overall performance. [sent-372, score-0.52]

98 We asked 4 annotators to rate DUC2007 predicted summaries (45 summary pairs per annotator). [sent-373, score-0.373]

99 Kz candidate sentence scores candidate sentence scores . [sent-403, score-0.356]

100 We demonstrated that implementation of a summary focused hierarchical topic model to discover sentence structures as well as construction of a discriminative method for inference can benefit summarization quality on manual and automatic evaluation metrics. [sent-407, score-0.779]


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