CS474 Intro to Natural Language Question answering Processing • Overview and task definition • History • Open-domain question answering Question Answering • Basic system architecture • Predictive indexing methods – Slides based on those of Jamie Callan, CMU • Pattern-matching methods • Advanced techniques Indexing with predictive annotation Predictive annotation • Some answers belong to well-defined semantic classes – People, places, monetary amounts, telephone numbers, addresses, organizations • Predictive annotation: index a document with “concepts” or “features” that are expected to be useful in (many) queries – E.g. people names, location names, addresses, etc. • Add additional operators for use in queries – E.g. Where does Ellen Vorhees work? “Ellen Vorhees” NEAR/10 *organization Predictive annotation Issues for predictive annotation • How is annotated text stored in the index? • What makes a good QA-token? – Question that would use the token • Can be recognized with high reliability (high precision) • Occurs frequently enough to be worth the effort • How do you want the system to make use of the QA-tokens? – Filtering step? – Transform original question into an ad-hoc retrieval question that incorporates QA-tokens and proximity operators? • Common approaches to recognizing QA-tokens • Treat <$QA-token, term> as meaning that $QA-token – Tables, lists, dictionaries and term occur at the same location in the text – Heuristics – Or use phrase indexing approach to index as a single – Hidden Markov models item Advantages and disadvantages Question answering + Most of the computational cost occurs during indexing • Overview and task definition • Allows use of more sophisticated methods • History + Annotator has access to complete text of document • Important for recognizing some types of features • Open-domain question answering – Must know ahead of time which types of • Basic system architecture features/concepts are likely to be important – Increases size of index considerably • Predictive indexing methods • E.g. by an order of magnitude if many features • Pattern-matching methods – Slides based on those of Jamie Callan, CMU • Used (in varying amounts) by almost all open-domain • Advanced techniques Q/A systems Simple pattern-based QA Example • Observation: there are many • Who is the President of Cornell? questions…but fewer types of questions – Expectation: answer string contains a • Each type of question can be associated person name with • Named entity identification – Expectations about answer string – Search query: “president Cornell characteristics *PersonName” – Strategies for retrieving documents that – Rule: “*PersonName, President of Cornell” might have answers • Matches “…Hunter Rawlings, President of –Rulesfor identifying answer strings in Cornell” documents • Answer = “Hunter Rawlings” Question analysis Question analysis example • Input: the question • “Who is Elvis?” • Output – Question type: “who” – Named-entity tagging: “Who is <person- – Search query name>Elvis</person-name>” – Answer expectations – Analysis pattern: if question type = “who” and question – Extraction strategy contains <person-name> then • Search query doesn’t need to contain a *PersonName operator • Requires • Desired answer probably is a description – Identifying named entities • Likely answer extraction patterns – Categorizing the question – “Elvis, the X” » “…Elvis, the king of rock and roll…” – Matching question parts to templates –“the X Elvis” • Method: pattern-matching » “the legendary entertainer Elvis” – Analysis patterns created manually these days… Question analysis Example: What is Jupiter? Frequency of question Relative difficulty of 1. What We Will Learn from Galileo 2. The Nature of Things: Jupiter’s shockwaves—How a comet’s types on an Internet question types bombardment has sparked activity on Earth search engine –Whatis difficult 3. Jupiter-Bound Spacecraft Visits Earth on 6-Year Journey – 42% what – What time… 4. STAR OF THE MAGI THEORIES ECLIPSED? 5. Marketing & Media: Hearst, Burda to Scrap New Astrology – 21% where – What country… Magazine – 20% who 6. Greece, Italy Conflict On Cause Of Ship Crash That Kills 2, –Whereis easy Injures 54 –8% when –Whois easy 7. Interplanetary Spacecraft To `Visit` Earth With LaserGraphic – 8% why –Whenis easy 8. A List of Events During NASA’s Galileo Mission to Jupiter 9. SHUTTLE ALOFT, SENDS GALILEO ON 6-YEAR VOYAGE – 2% which –Whyis hard TO JUPITER – 0% how –Whichis hard 10. Rebuilt Galileo Probe readied For Long Voyage To Jupiter –Howis hard Simple pattern-based Q/A: Answer extraction assessment • Select highly ranked sentences from highly ranked documents • Extremely effective when • Perform named-entity tagging (or extract from index) – Question patterns are predictable and perform part of speech tagging • Fairly “few” patterns cover the most likely questions – Could be several hundred – “The/DT planet/NN <location>Jupiter/NNP</location> • Not much variation in vocabulary and/CC its/PRP moons/NNS are/VBP in/IN effect/NN – Simple word matching works a/DT mini-solar/JJ system/NN ,/, and/CC • The corpus is huge (e.g., Web) <location>Jupiter/NNP</location> itself/PRP is/VBZ – Odds of finding an answer document that matches the often/RB called/VBN a/DT star/NN that/IN never/RB vocabulary and answer extraction rule improves caught/VBN fire/NN ./.” • Somewhat labor intensive • Apply extraction patterns – Patterns are created and tested manually – the/DT X Y, Y=Jupiter -> the planet Jupiter -> “planet” Common problem: matching Common problem: matching questions to answers questions to answers • Document word order isn’t exactly what was • Answer vocabulary doesn’t exactly match expected question vocabulary • Solution: “soft matching” of answer patterns to • Solution: bridge the vocabulary mismatch document text – Approach: use WordNet to identify simple – Approach: use distance-based answer selection relationships when no rule matches • E.g. for “What is Hunter Rawlings’ address?” • “astronaut” is a type of “person” – Use the address nearest to the words “Hunter • “astronaut” and “cosmonaut” are synonyms Rawlings” – User the address in the same sentence as “Hunter Rawlings” Common problem: improving the Common problem: improving set of retrieved documents answer extraction patterns • Sometimes the IR system can’t find any documents that • Word sequence patterns have limited power have answers (even though the right documents are in the corpus) • Solution: create patterns that use syntactic information • Solution: get a broader set of documents – Partial syntactic parsing of documents – Approach: if answer extractor fails to find an answer, • Is this noun the subject or the object of the sentence? kick the question back to the search engine with – Allows more complex patterns instructions to widen the search • Question: “Who shot Kennedy?” • Assumes answer extractors can tell when they fail • “Who” implies a person that should be subject of answer – Approach: use a variety of retrieval strategies to retrieve sentence/clause documents • “Kennedy” should be direct object of answer • E.g., all words within one sentence, then all words within one • Pattern: <subject> shot Kennedy paragraph, then within same document, … • Matching text: “Oswald shot Kennedy” • E.g. add synonyms to query or do query expansion • Simple, but much higher computational expense Common problem: Question answering selecting/ranking the answer • Overview and task definition • Multiple answer candidates • Solutions • History – Features used to represent answer candidates • Open-domain question answering • Frequency • Basic system architecture • Distance to question words • Predictive indexing methods • Location in answer passage(s) •… • Pattern-matching methods – Selection functions • Advanced techniques • Created manually – Slides based on Pasca & Harabagiu (SIGIR 2001) and the slides of J. Callan and K. Czuba, CMU • Learned from training data SMU/LCC system Multi-strategy approach • State of the art in QA is the SMU/LCC Falcon system – Employs informed use of standard IR techniques – Use of broad ontology (extended WordNet) TREC 2001 – Lots of NLP – Answer verification • Similar to most other systems in architecture TREC 2000 except for – Much more careful tuning of algorithms and resources – More sophisticated control of IR and NLP – Feedback loops Question analysis Expected answer types • Parsing and named entity recognition • Expected answer type determined by parsing • Exceptions for “special cases” Expected answer types Paragraph retrieval • Answer types are mapped to named-entity • Boolean retrieval with loops categories that can be recognized in text – Different from multiple queries in that system only uses additional queries when necessary – Fewer candidates for analysis components to consider • Loop 1: query keyword loop – Keywords added/dropped to make query more/less specific • Loop 2: keyword alternations – Try morphological variants and synonyms • Answer types drive processing of paragraphs • Loop 3: semantic alternations – Passages need to contain the expected answer type – Try semantic alternatives Feedback loops Answer verification • Parse passages to create a dependency tree among words • Attempt to unify logical forms of question and answer text Assessment • Strengths – Controlled use of IR system • Query expansion via lexical and semantic equivalents • Believed to be the major power of the system – Tailored resources (see paper) • WordNet, parser, NE identifier, etc. – Answer verification • Initially thought to be the key component of the system •Now…not so clear • Weaknesses – Complex system, contribution of each component unclear.