# A Maximum Entropy-based Model for Answer Extraction Dan Shen IGK, Saarland University Supervisors:Prof. Dietrich Klakow Dr. ir. Geert-Jan M. Kruijff.

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A Maximum Entropy-based Model for Answer Extraction Dan Shen IGK, Saarland University Supervisors:Prof. Dietrich Klakow Dr. ir. Geert-Jan M. Kruijff

Part I -- Introduction Answer Extraction Module in QA Statistical Method for Answer Extraction  Motivation  Framework

Answer Extraction Module in QA Open-Domain factoid Question Answering Open-Domain factoid Question Answering Basic modules Basic modules  Information Retrieval Module  a set of relevant sentences / paragraphs  Answer Extraction (AE) Module  the appropriate answer phrase Q: What is the capital of Japan ? A: Tokyo Q: How far is it from Earth to Mars ? A: 249 million miles

Techniques and Resources for AE  How to incorporate them ?  Pipeline structure  Mathematical framework TechniquesResources Pattern Matching NER Parsing Semantic analysis Reasoning ….. WordNet Web Database Ontology …

Motivation – Use Statistical Methods ? Flexibility Flexibility  Integrating various techniques / resources  Easy to extend to span more in the future Effectiveness

1899: What book did Rachel Carson write in 1962 ? 1. Rachel Carson 's 1962 book Silent Spring said dieldrin causes mania. 2. It could almost be said that what we call the environmental movement did begin with a single book : Rachel Carson 's Silent Spring. 3. That caused Rachel Carson to write in her 1962 book Silent Spring that their disappearance might make it necessary for us to find a new national emblem. 4. 1962 : Rachel Carson writes Silent Spring, the first shot in the war against environmental pollution, particularly DDT. 6. In 1962, former U.S. Fish and Wildlife Service biologist Rachel Carson shocked the nation with her landmark book, Silent Spring. 7. In a forward to the new book, U.S. Vice-President Albert Gore described the book critically important and compared it with Silent Spring, Rachel Carson 's 1962 book that set off a movement to ban DDT and other pesticides. ……

Research Issues Answer Candidate Selection Answer Candidate Selection  Which constituent is regarded as an AC ? Methods Methods  classification / ranking / … Features Features

Part II – ME-based model Method Features Experiments and Results

Part II – ME-based model Method Features Experiments and Results

Maximum Entropy Formulation I Given a set of answer candidates Model the probability Define Features Functions Decision Rule

Maximum Entropy Formulation II Given a set of answer candidates Model the probability Define Features Functions Decision Rule

Some Considerations Model I Model I  Judge whether each candidate is a correct answer  √ Can find more than one correct answer in a sentence  ? Is the probability comparable ?  × Suffer from the unbalanced data set (1Pos / >20Neg) Model II  Find the best answer among the candidates  × In a sentence, it just find one correct answer  √ Directly make the probabilities of the candidates comparable Experiment Experiment  Model II outperform Model I by about 5%

Part II – ME-based model Method Features Experiments and Results

Question Analysis Q: What US biochemists won the Nobel Prize in medicine in 1992 ? Question Word -- what Target Word – biochemist Subject Word -- Nobel Prize / medicine / 1992 Verb – win Q: What is the name of the highest mountain in Africa ? Question Word -- what Target Word -- mountain Subject Words -- highest / Africa Verb -- be PERSON LOCATION

Answer Candidate Selection Preprocessing Preprocessing  Named Entity Recognition  Parsing [Collins Parser]  To dependency tree Answer Candidate Selection Answer Candidate Selection  Base noun phrase  Named entities  Leaf nodes Answer Candidate Coverage Answer Candidate Coverage  11876 / 14039 = 84.6 %  Missing some sentences  to consider all of the nodes ?

Features – Syntactic / POS Tag Features Observation Observation  For who / where Question, answers = Proper Noun  For how / when Question, answers = CD Question Word × Syntactic tag / Pos tag Question Word × Syntactic tag / Pos tag  QWord = “how” & SynTag = “CD”  QWord = “who” & SynTag = “NNP”  QWord = “when” & SynTag = “NNP”  QWord = “when” & SynTag = “CD”  …

Features – Surface Word Features Word formations Word formations  Length / Capitalized / Digits, …  Question Word × Word formations QWord = “who” & word is capitalized QWord = “who” & word length < 3 Words co-occurrence between Q and A Words co-occurrence between Q and A  Observation -- Answer aren’t a subsequence of question

Features – Named Entity Features Question Type × NE type Question Type × NE type  QType = Person & NE type = Person  QType = Date & NE type = Date  QType = how much & NE type = Money  … Useful for who, where, when … Question Useful for who, where, when … Question But for What / Which / How questions ? But for What / Which / How questions ?  Many expected answer types not belong to a defined NE type Q1: What language is most commonly used in Bombay ? Q2: What city is … Q3: Which movie win ….

Features – TWord Relation for WHAT I TWord is a hypernym of answer TWord is a hypernym of answer TWord is the head of answer TWord is the head of answer Q: What is the name of the airport in Dallas Ft. Worth ? A: Wednesday morning, the low temperature at the Dallas-Fort Worth International Airport was 81 degrees. Q: What city is Disneyland in ? A: Not bad for a struggling actor who was working at Tokyo Disneyland just a few years ago.

Features – TWord Relation for WHAT II TWord is the Appositive of answer TWord is the Appositive of answer Feature Function Feature Function  QWord = what & TWord is hypernym of answer candidate  … Q: What book did Rachel Carson write in 1962 ? A1: In her 1962 book Silent Spring, Rachel Carson, a marine biologist, chronicled DDT 's poisonous effects, …. A2: In 1962, former U.S. Fish and Wildlife Service biologist Rachel Carson shocked the nation with her landmark book, Silent Spring.

Features – Tword Relation for HOW How many / much + NN … How many / much + NN … How long / far / tall / fast … How long / far / tall / fast …  How long …  year / day / month / …  How tall …  feet / inch / mile / …  How fast …  per day / per hour / …  Use some trigger word features Q: How many time zones are there in the world ? A: The world is divided into 24 time zones.

Features – Subject Word Relations I Q: Who invented the paper clip ? S1: The paper clip, weighing a desk-crushing 1320 pounds, is a faithful copy of Norwegian Johan Vaaler ‘s 1899 invention, said … S2: “ Like the guy who invented the safety pin, or the guy who invented the paper clip “, David says. ×

Features – Subject Word Relations II Match subject word in the answer sentence Match subject word in the answer sentence  Minimal Edit Distance Dependency Relationship Matching Dependency Relationship Matching  Observation – answer are close to SWord in Dependency Tree  answer and SWord have some relation  Answer candidate is a subject word  Answer candidate is the parent / child / brother of SWord  The path from the answer candidate to SWord Q: What is the name of the airport in Dallas Ft. Worth ? A: Wednesday morning, the low temperature at the Dallas-Fort Worth International Airport was 81 degrees

Part II – ME-based model Method Features Experiments and Results

Experiment Settings Training Data Training Data  TREC 1999, TREC 2000, TREC 2002  Total Number of Questions: 1108  Total Number of Sentences: 11331 Test Data Test Data  TREC 2003  Total Number of Questions: 362 (remove NIL question)  Total Number of Sentences: 2708

Question Word Distribution

Overall Performance WhoWhenWhereWhat MRR0.750.74510.609 WhichHowWhyOther MRR10.50800 Overall MRR 0.60 MRR – Mean Reciprocal Rank return five answers for each question

Contribution of Different Features

Features – Syntactic / POS Tag Features

Features – + Surface Word Features

Features – + Named Entity Features

Features – + TWord Relations for WHAT

Features – + TWord Relations for HOW

Features – + Subject Word Relations

Error Analysis – I Target Word Concept Unresolved Target Word Concept Unresolved  Q: What is the traditional dish served at Wimbledon? √ A: And she said she wasn't wild about Wimbledon 's famed strawberries and cream. ×A: And she said she wasn't wild about Wimbledon 's famed strawberries and cream. Choosing the Wrong Entity Choosing the Wrong Entity  Q: What actress has received the most Oscar nominations? √ A: Oscar perennial Meryl Streep is up for best actress for the film, tying Katharine Hepburn for most acting nominations with 12. ×A: Oscar perennial Meryl Streep is up for best actress for the film, tying Katharine Hepburn for most acting nominations with 12.

Error Analysis – II Answer Candidate Granularity Answer Candidate Granularity  Q: What city is Disneyland in? √ A: Not bad for a struggling actor who was working at Tokyo Disneyland just a few years ago. ×A: Not bad for a struggling actor who was working at Tokyo Disneyland just a few years ago. Repeated Target Word in Answer Repeated Target Word in Answer  Q: How many grams in an ounce? √ A: NOTE : 30 grams is about 1 ounce. ×A: NOTE : 30 grams is about 1 ounce. Misc. Misc.

Future Work Extract answer from Web Evaluate on other data sets  Knowledge Master Corpus How to deal with NIL question ? Incorporate more linguistic-motivated features

The End

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