Exploiting Timelines to Enhance Multi-document Summarization

Jun-Ping Ng, Yan Chen, Min-Yen Kan, Zhoujun LiDSO National Laboratories

National University of SingaporeBeihang University

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Outline

• Overview• Approach• Experiments and Results • Discussion

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OVERVIEW

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Multi-document Summarization

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Extractive Summarization

• Find the most salient sentences in source collection

• Top-k sentences are extracted to compose final summary

• <Graphic>

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Two Storms

(1) A fierce cyclone packing extreme winds and torrential rain smashed into Bangladesh’s southwestern coast Thursday, wiping out homes and trees in what officials described as the worst storm in years.

(2) More than 100,000 coastal villagers have been evacuated before the cyclone made landfall.

(3) The storm matched one in 1991 that sparked a tidal wave that killed an estimated 138,000 people, Karmakar told AFP

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Two Storms

(1) A fierce cyclone packing extreme winds and torrential rain smashed into Bangladesh’s southwestern coast Thursday, wiping out homes and trees in what officials described as the worst storm in years.

(2) More than 100,000 coastal villagers have been evacuated before the cyclone made landfall.

(3) The storm matched one in 1991 that sparked a tidal wave that killed an estimated 138,000 people, Karmakar told AFP

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Timeline

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APPROACH

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Merging Timelines Into Summarization

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Temporal Processing

• Based on TimeML (Pustejovsky et al 2003)• Basic temporal units – events + timexes• Three steps

– Event-timex temporal relation classification– Event-event temporal relation classification– Timex normalization

• Merge to obtain timelines• <TODO>

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Timelines

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Summarization --- SWINGhttps://github.com/WING-NUS/SWING

Sentence Scoring• Time span importance• Contextual time span importance• Sentence temporal coverage density

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Defining Timeline Features

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Time Span Importance (TSI)• Time spans which contain many events are more salient• Sentences which references events in these time spans are

thus better candidates for a summary

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Scoring TSI

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Contextual Time Span Importance (CTSI)

• Time spans near to “important” time spans may also be important

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Scoring CTSI

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Sentence Temporal Coverage Density (TCD)

• Number of sentences in a summary is limited• Favour sentences which

– contain more events– covering a wide variety of time spans

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Scoring TCD

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Sentence Re-ordering• SWING makes use of the Maximal Marginal Relevance

(MMR) algorithm to identify redundancies in selected sentences

• MMR is heavily biased towards lexicons and surface similarities

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Beyond Lexical Penalties

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An official in Barisal, 120 kilometres south of Dhaka, spoke of severe destruction as the 500 kilometre-wide mass of cloud passed overhead.

“Many trees have been uprooted and houses and schools blown away,” Mostofa Kamal, a district relief and rehabilitation officer, told AFP by telephone.

“Mud huts have been damaged and the roofs of several houses blown off,” said the state’s relief minister, Mortaza Hossain.

TimeMMR• Novel dimension to redundancy detection• Beyond lexical similarities, identify sentences which contain

substantial time span overlaps• Candidate sentences which share many time spans with

selected sentences are penalised

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EXPERIMENTS AND RESULTS

Results• TAC-2010 data set to

train regression model• TAC-2011 data set to

test • Using timelines lead to

better summaries!

System ROUGE-2

SWING 0.1339

+ Timelines 0.1394*

+ TimeMMR 0.1389

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Overcoming Errors• Timelines contain errors

– Errors from underlying temporal processing systems– Simplifying assumptions made in timeline construction– Lack of consistency checking and validation

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Reliability Filtering• Identify timelines which potentially contain more errors• Exclude these when performing summarization

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Length as a Metric• Use the length of a timeline as a gauge of its “accuracy”• Drop the use of timelines which are less than the average

length, computed over the whole input document collection

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Results• Experiments repeated

with reliability filtering• Significant

improvement obtained • After filtering timelines

are used in 21 out of 44 document sets

System ROUGE-2

SWING 0.1339

+ Timelines 0.1394*

+ Timelines + Filtering 0.1418**

+ TimeMMR 0.1389

+ TimeMMR+ Filtering 0.1402**

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DISCUSSION

Text Example

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The Army’s surgeon general criticized stories in The Washington Post disclosing problems at Walter Reed Army Medical Center, saying the series unfairly characterized the living conditions and care for soldiers recuperating from wounds at the hospital’s facilities.

The Army’s surgeon general criticized stories in The Washington Post disclosing problems at Walter Reed Army Medical Center, saying the series unfairly characterized the living conditions and care for soldiers recuperating from wounds at the hospital’s facilities.

Defense Secretary Robert Gates says people found to have been responsible for allowing substandard living conditions for soldier outpatients at Walter Reed Army Medical Center in Washington will be “held account- able,” although so far no one in the Army chain of com- mand has offered to resign.

A top Army general vowed to personally over- see the upgrading of Walter Reed Army Medical Cen- ter’s Building 18, a dilapidated former hotel that houses wounded soldiers as outpatients.

Top Army officials visited Building 18, the decrepit former hotel housing more than 80 recovering soldiers, outside

“I’m not sure it was an accurate representation,” Lt. Gen. Kevin Kiley, chief of the Army Medical Com- mand which oversees Walter Reed and all Army health care, told reporters during a news conference.

Timelines Used SWING

Future Work• Study the use of alternative evaluation metrics, especially

for TimeMMR• Look at better metrics for reliability filtering• Expand the scope of the timelines that are used for more

flexibility

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Conclusion

• The use of time is useful for summarization!• Sentence Scoring

– Derive features from a timeline– Combine features with a supervised learning

summarization framework• Sentence Re-ordering

– Use overlapping time spans to identify redundancies

Thank you!

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