CSC 8520 Spring Paula Matuszek CS 8520: Artificial Intelligence Natural Language Processing Introduction Paula Matuszek Spring, 2010

2 CSC 8520 Spring Paula Matuszek Natural Language Processing speech recognition natural language understanding computational linguistics psycholinguistics information extraction information retrieval inference natural language generation speech synthesis language evolution

3 CSC 8520 Spring Paula Matuszek Applied NLP Machine translation spelling/grammar correction Information Retrieval Data mining Document classification Question answering, conversational agents

4 CSC 8520 Spring Paula Matuszek Natural Language Understanding accoustic /phonetic morphological/synt actic semantic / pragmatic sound waves internal representation

5 CSC 8520 Spring Paula Matuszek Sounds Symbols Sense Natural Language Understanding accoustic /phonetic morphological/synta ctic semantic / pragmatic sound waves internal representation

6 CSC 8520 Spring Paula Matuszek “How to recognize speech, not to wreck a nice beach” “The cat scares all the birds away” “The cat’s cares are few” Where are the words? sound waves internal representation accoustic /phonetic morphological/synt actic semantic / pragmatic - pauses in speech bear little relation to word breaks + intonation offers additional clues to meaning

7 CSC 8520 Spring Paula Matuszek “The dealer sold the merchant a dog” “I saw the Golden bridge flying into San Francisco” Word creation: establish establishment the church of England as the official state church. disestablishment antidisestablishment antidisestablishmentarian antidisestablishmentarianism is a political philosophy that is opposed to the separation of church and state. Dissecting words/sentences internal representation accoustic /phonetic morphological/synt actic semantic / pragmatic sound waves

8 CSC 8520 Spring Paula Matuszek “I saw Pathfinder on Mars with a telescope” “Pathfinder photographed Mars” “The Pathfinder photograph from Ford has arrived” “When a Pathfinder fords a river it sometimes mars its paint job.” What does it mean? sound waves internal representation accoustic /phonetic morphological/ syntactic semantic / pragmatic

9 CSC 8520 Spring Paula Matuszek What does it mean? sound waves internal representation accoustic /phonetic morphological/ syntactic semantic / pragmatic He Heit “Jack went to the store. He found the milk in aisle 3. He paid for it and left.” “ Q: Did you read the report? A: I read Bob’s .”

CSC 8520 Spring Paula Matuszek 19 The steps in NLP (Cont.) Morphology: Concerns the way words are built up from smaller meaning bearing units. Syntax: concerns how words are put together to form correct sentences and what structural role each word has Semantics: concerns what words mean and how these meanings combine in sentences to form sentence meanings Taken from ocw.kfupm.edu.sa/user062/ICS48201/NL2introduction.ppt

CSC 8520 Spring Paula Matuszek 2/20/2007 Taken from ocw.kfupm.edu.sa/user062/ICS48201/NL2introduction.ppt 20 The steps in NLP (Cont.) Pragmatics: concerns how sentences are used in different situations and how use affects the interpretation of the sentence Discourse: concerns how the immediately preceding sentences affect the interpretation of the next sentence

12 CSC 8520 Spring Paula Matuszek Some of the Tools Regular Expressions and Finite State Automata Part of Speech taggers N-Grams Grammars Parsers Semantic Analysis

CSC 8520 Spring Paula Matuszek 2/20/2007 Taken from ocw.kfupm.edu.sa/user062/ICS48201/NL2introduction.ppt 21 Parsing (Syntactic Analysis) Assigning a syntactic and logical form to an input sentence uses knowledge about word and word meanings (lexicon) uses a set of rules defining legal structures (grammar) Paula ate the apple. (S (NP (NAME Paula)) (VP (V ate) (NP (ART the) (N apple))))

CSC 8520 Spring Paula Matuszek 2/20/2007 Taken from ocw.kfupm.edu.sa/user062/ICS48201/NL2introduction.ppt 22 Word Sense Resolution Many words have many meanings or senses We need to resolve which of the senses of an ambiguous word is invoked in a particular use of the word I made her duck. (made her a bird for lunch or made her move her head quickly downwards?)

CSC 8520 Spring Paula Matuszek 2/20/2007 Taken from ocw.kfupm.edu.sa/user062/ICS48201/NL2introduction.ppt 23 Reference Resolution Domain Knowledge (Registration transaction) Discourse Knowledge World Knowledge U: I would like to register in an CSC Course. S: Which number? U: Make it S: Which section? U: Which section is in the evening? S: section 1. U: Then make it that section.

CSC 8520 Spring Paula Matuszek 2/20/2007 Taken from ocw.kfupm.edu.sa/user062/ICS48201/NL2introduction.ppt 28 Morphological Analysis Syntactic Analysis Semantic Analysis Discourse Analysis Pragmatic Analysis Internal representation lexicon user Surface form Perform action stems parse tree Resolve references Stages of NLP

17 CSC 8520 Spring Paula Matuszek Human Languages You know ~50,000 words of primary language, each with several meanings six year old knows ~13000 words First 16 years we learn 1 word every 90 min of waking time Mental grammar generates sentences -virtually every sentence is novel 3 year olds already have 90% of grammar ~6000 human languages – none of them simple! Adapted from Martin Nowak 2000 – Evolutionary biology of language – Phil.Trans. Royal Society London

18 CSC 8520 Spring Paula Matuszek Human Spoken language Most complicated mechanical motion of the human body –Movements must be accurate to within mm –synchronized within hundredths of a second We can understand up to 50 phonemes/sec (normal speech 10-15ph/sec) –but if sound is repeated 20 times /sec we hear continuous buzz! All aspects of language processing are involved and manage to keep apace Adapted from Martin Nowak 2000 – Evolutionary biology of language – Phil.Trans. Royal Society London

19 CSC 8520 Spring Paula Matuszek Why Language is Hard NLP is AI-complete Abstract concepts are difficult to represent LOTS of possible relationships among concepts Many ways to represent similar concepts Tens of hundreds or thousands of features/dimensions

20 CSC 8520 Spring Paula Matuszek Why Language is Easy Highly redundant Many relatively crude methods provide fairly good results

21 CSC 8520 Spring Paula Matuszek History of NLP Prehistory (1940s, 1950s) –automata theory, formal language theory, markov processes (Turing, McCullock&Pitts, Chomsky) –information theory and probabilistic algorithms (Shannon) –Turing test – can machines think? Early work: –symbolic approach generative syntax - eg Transformations and Discourse Analysis Project (TDAP- Harris) AI – pattern matching, logic-based, special-purpose systems –Eliza Rogerian therapist –stochastic baysian methods early successes  $$$$ grants! by 1966 US government had spent 20 million on machine translation alone Critics: –Bar Hillel – “no way to disambiguation without deep understanding” –Pierce NSF 1966 report: “no way to justify work in terms of practical output”

22 CSC 8520 Spring Paula Matuszek History of NLP The middle ages ( ) –stochastic speech recognition and synthesis (Bell Labs) –logic-based compositional semantics (Montague) definite clause grammars (Pereira&Warren) –ad hoc AI-based NLU systems SHRDLU robot in blocks world (Winograd) knowledge representation systems at Yale (Shank) –discourse modeling anaphora focus/topic (Groz et al) conversational implicature (Grice)

23 CSC 8520 Spring Paula Matuszek History of NLP NLP Renaissance ( ) Lessons from phonology & morphology successes: –finite-state models are very powerful –probabilistic models pervasive –Web creates new opportunities and challenges –practical applications driving the field again 21 st Century NLP The web changes everything: –much greater use for NLP –much more data available

24 CSC 8520 Spring Paula Matuszek Information Retrieval

25 CSC 8520 Spring Paula Matuszek Finding Out About There are many large corpora of information that people use. The web is the obvious example. Others include: –scientific journals –patent databases –Medline –Usenet groups People interact with all that information because they want to KNOW something; there is a question they are trying to answer or a piece of information they want. Information Retrieval, or IR, is the process of answering that information need. Simplest approach: –Knowledge is organized into chunks (pages or documents) –Goal is to return appropriate chunks

26 CSC 8520 Spring Paula Matuszek Information Retrieval Systems Goal of an information retrieval system is to return appropriate chunks Steps involve include –asking a question –finding answers –evaluating answers –presenting answers Value of an IR tool depends on how well it does on all of these. Web search engines are the IR tools most familiar to most people.

27 CSC 8520 Spring Paula Matuszek Asking a question Reflect some information need Query Syntax needs to allow information need to be expressed –Keywords –Combining terms Simple: “required”, NOT (+ and -) Boolean expressions with and/or/not and nested parentheses Variations: strings, NEAR, capitalization. –Simplest syntax that works –Typically more acceptable if predictable Another set of problems when information isn’t text: graphics, music

28 CSC 8520 Spring Paula Matuszek Finding the Information Goal is to retrieve all relevant chunks. Too time- consuming to do in real-time, so IR systems index pages. Two basic approaches –Index and classify by hand –Automate For BOTH approaches deciding what to index on (e.g., what is a keyword) is a significant issue. Many IR tools like search engines provide both

29 CSC 8520 Spring Paula Matuszek IR Basics A retriever collects a page or chunk. This may involve spidering web pages, extracting documents from a DB, etc. A parser processes each chunk and extracts individual words. An indexer creates/updates a hash table which connects words with documents A searcher uses the hash table to retrieve documents based on words A ranking system decides the order in which to present the documents: their relevance

30 CSC 8520 Spring Paula Matuszek How Good Is The IR? Information Retrieval systems are evaluated with two basic metrics: –Precision: What percent of document returned are actually relevant to the information need –Recall: what percent of documents relevant to information need are returned Can’t typically measure these exactly; usually based on test sets.

31 CSC 8520 Spring Paula Matuszek Selecting Relevant Documents Assume: –we already have a corpus of documents defined. –goal is to return a subset of those documents. –Individual documents have been separated into individual files Remaining components must parse, index, find, and rank documents. Traditional approach is based on the words in the documents (predates the web)

32 CSC 8520 Spring Paula Matuszek Extracting Lexical Features Process a string of characters –assemble characters into tokens (tokenizer) –choose tokens to index Standard lexical analysis problem Lexical Analyser Generator, such as lex

33 CSC 8520 Spring Paula Matuszek Lexical Analyser Basic idea is a finite state machine Triples of input state, transition token, output state Must be very efficient; gets used a LOT blank A-Z blank, EOF

34 CSC 8520 Spring Paula Matuszek Design Issues for Lexical Analyser Punctuation –treat as whitespace? –treat as characters? –treat specially? Case –fold? Digits –assemble into numbers? –treat as characters? –treat as punctuation?

35 CSC 8520 Spring Paula Matuszek Lexical Analyser Output of lexical analyser is a string of tokens Remaining operations are all on these tokens We have already thrown away some information; makes more efficient, but limits somewhat the power of our search

36 CSC 8520 Spring Paula Matuszek Stemming Additional processing at the token level –We covered earlier this semester Turn words into a canonical form: –“cars” into “car” –“children” into “child” –“walked” into “walk” Decreases the total number of different tokens to be processed Decreases the precision of a search, but increases its recall

37 CSC 8520 Spring Paula Matuszek Noise Words (Stop Words) Function words that contribute little or nothing to meaning Very frequent words –If a word occurs in every document, it is not useful in choosing among documents –However, need to be careful, because this is corpus-dependent Often implemented as a discrete list

38 CSC 8520 Spring Paula Matuszek Example Corpora We are assuming a fixed corpus. Some sample corpora: –Medline Abstracts – . Anyone’s . –Reuters corpus –Brown corpus Will contain textual fields, maybe structured attributes –Textual: free, unformatted, no meta-information. NLP mostly needed here –Structured: additional information beyond the content

39 CSC 8520 Spring Paula Matuszek Structured Attributes for Medline Pubmed ID Author Year Keywords Journal

40 CSC 8520 Spring Paula Matuszek Textual Fields for Medline Abstract –Reasonably complete standard academic English –Capturing the basic meaning of document Title –Short, formalized –Captures most critical part of meaning –Proxy for abstract

41 CSC 8520 Spring Paula Matuszek Structured Fields for To, From, Cc, Bcc Dates Content type Status Content length Subject (partially)

42 CSC 8520 Spring Paula Matuszek Text fields for Subject –Format is structured, content is arbitrary. –Captures most critical part of content. –Proxy for content -- but may be inaccurate. Body of –Highly irregular, informal English. –Entire document, not summary. –Spelling and grammar irregularities. –Structure and length vary.

43 CSC 8520 Spring Paula Matuszek Indexing We have a tokenized, stemmed sequence of words Next step is to parse document, extracting index terms –Assume that each token is a word and we don’t want to recognize any more complex structures than single words. When all documents are processed, create index

44 CSC 8520 Spring Paula Matuszek Basic Indexing Algorithm For each document in the corpus –Get the next token –Create or update an entry in a list doc ID, frequency. For each token found in the corpus –calculate #docs, total frequency –sort by frequency –Often called a “reverse index”, because it reverses the “words in a document” index to be a “documents containing words” index. –May be built on the fly or created after indexing.

45 CSC 8520 Spring Paula Matuszek Fine Points Dynamic Corpora (e.g., the web): requires incremental algorithms Higher-resolution data (eg, char position). –Supports highlighting –Supports phrase searching –Useful in relevance ranking Giving extra weight to proxy text (typically by doubling or tripling frequency count) Document-type-specific processing –In HTML, want to ignore tags –In , maybe want to ignore quoted material

46 CSC 8520 Spring Paula Matuszek Choosing Keywords Don’t necessarily want to index on every word –Takes more space for index –Takes more processing time –May not improve our resolving power How do we choose keywords? –Manually –Statistically Exhaustivity vs specificity

47 CSC 8520 Spring Paula Matuszek Manually Choosing Keywords Unconstrained vocabulary: allow creator of document to choose whatever he/she wants –“best” match –captures new terms easily –easiest for person choosing keywords Constrained vocabulary: hand-crafted ontologies –can include hierarchical and other relations –more consistent –easier for searching; possible “magic bullet” search

48 CSC 8520 Spring Paula Matuszek Examples of Constrained Vocabularies ACM headings ( H: Information Retrieval –H3: Information Storage and Retrieval – H3.3: Information Search and Retrieval »Clustering »Query formulation »Relevance feedback »Search process etc. Medline Headings ( L: Information Science –L01: Information Science – L01.700: Medical Informatics – L : Medical Informatics Applications – L : Information Storage and Retrieval »Grateful Med [L ]

49 CSC 8520 Spring Paula Matuszek Automated Vocabulary Selection Frequency: Zipf’s Law. –P n = 1/n a, where Pn is the frequency of occurrence of the nth ranked item and a is close to 1 –Within one corpus, words with middle frequencies are typically “best” Document-oriented representation bias: lots of keywords/document Query-Oriented representation bias: only the “most typical” words. Assumes that we are comparing across documents.

50 CSC 8520 Spring Paula Matuszek Choosing Keywords “Best” depends on actual use; if a word only occurs in one document, may be very good for retrieving that document; not, however, very effective overall. Words which have no resolving power within a corpus may be best choices across corpora Not very important for web searching; more relevant for some text mining.

51 CSC 8520 Spring Paula Matuszek Keyword Choice for WWW We don’t have a fixed corpus of documents New terms appear fairly regularly, and are likely to be common search terms Queries that people want to make are wide- ranging and unpredictable Therefore: can’t limit keywords, except possibly to eliminate stop words. Even stop words are language-dependent. So determine language first.

52 CSC 8520 Spring Paula Matuszek Comparing and Ranking Documents Once our IR system has retrieved a set of documents, we may want to Rank them by relevance –Which are the best fit to my query? –This involves determining what the query is about and how well the document answers it Compare them –Show me more like this. –This involves determining what the document is about.

53 CSC 8520 Spring Paula Matuszek Determining Relevance by Keyword The typical document retrieval query consists entirely of keywords. Retrieval can be binary: present or absent More sophisticated is to look for degree of relatedness: how much does this document reflect what the query is about ? Simple strategies: –How many times does word occur in document? –How close to head of document? –If multiple keywords, how close together?

54 CSC 8520 Spring Paula Matuszek Keywords for Relevance Ranking Count: repetition is an indication of emphasis –Very fast (usually in the index) –Reasonable heuristic –Unduly influenced by document length –Can be "stuffed" by web designers Position: Lead paragraphs summarize content –Requires more computation –Also reasonably heuristic –Less influenced by document length

55 CSC 8520 Spring Paula Matuszek Keywords for Relevance Ranking Proximity for multiple keywords –Requires even more computation –Obviously relevant only if have multiple keywords –Effectiveness of heuristic varies with information need; typically either excellent or not very helpful at all All keyword methods –Are computationally simple and adequately fast –Are effective heuristics –typically perform as well as in-depth natural language methods for standard IR

56 CSC 8520 Spring Paula Matuszek Comparing Documents " Find me more like this one " really means that we are using the document as a query. This requires that we have some conception of what a document is about overall. Depends on context of query. We need to –Characterize the entire content of this document –Discriminate between this document and others in the corpus

57 CSC 8520 Spring Paula Matuszek Characterizing a Document: Term Frequency A document can be treated as a sequence of words. Each word characterizes that document to some extent. When we have eliminated stop words, the most frequent words tend to be what the document is about Therefore: f kd (# of occurrences of word K in document d) will be an important measure. Also called the term frequency

58 CSC 8520 Spring Paula Matuszek Characterizing a Document: Document Frequency What makes this document distinct from others in the corpus? The terms which discriminate best are not those which occur with high frequency! Therefore: D k (# of documents in which word K occurs) will also be an important measure. Also called the document frequency

59 CSC 8520 Spring Paula Matuszek TF*IDF This can all be summarized as: –Words are best discriminators when they occur often in this document (term frequency) don’t occur in a lot of documents (document frequency) One very common measure of the importance of a word to a document is TF*IDF : term frequency * inverse document frequency There are multiple formulas for actually computing this. The underlying concept is the same in all of them.

60 CSC 8520 Spring Paula Matuszek Describing an Entire Document So what is a document about ? TF*IDF: can simply list keywords in order of their TF*IDF values Document is about all of them to some degree: it is at some point in some vector space of meaning

61 CSC 8520 Spring Paula Matuszek Vector Space Any corpus has defined set of terms (index) These terms define a knowledge space Every document is somewhere in that knowledge space -- it is or is not about each of those terms. Consider each term as a vector. Then –We have an n-dimensional vector space –Where n is the number of terms (very large!) –Each document is a point in that vector space The document position in this vector space can be treated as what the document is about.

62 CSC 8520 Spring Paula Matuszek Similarity Between Documents How similar are two documents? –Measures of association How much do the feature sets overlap? Modified for length: DICE coefficient –DICE( x,y ) = 2 f( x,y ) / ( f( x ) + f( y ) ) –# terms compared to intersection Simple Matching coefficient: take into account exclusions –Cosine similarity similarity of angle of the two document vectors not sensitive to vector length

63 CSC 8520 Spring Paula Matuszek Bag of Words All of these techniques are what is known as bag of words approaches. Keywords treated in isolation Difference between "man bites dog" and "dog bites man" non-existent If better discrimination is needed, IR systems can add semantic tools –Use POS –Parse into basic NP VP structure –Requires that query be more complex.

64 CSC 8520 Spring Paula Matuszek Improvements The two big problems with short queries are: –Synonymy: Poor recall results from missing documents that contain synonyms of search terms, but not the terms themselves –Polysemy/Homonymy: Poor precision results from search terms that have multiple meanings leading to the retrieval of non-relevant documents. Martin:

65 CSC 8520 Spring Paula Matuszek Query Expansion Find a way to expand a users query to automatically include relevant terms (that they should have included themselves), in an effort to improve recall –Use a dictionary/thesaurus –Use relevance feedback Martin:

66 CSC 8520 Spring Paula Matuszek Dictionary/Thesaurus Example

67 CSC 8520 Spring Paula Matuszek Relevance Feedback Ask user to identify a few documents which appear to be related to their information need Extract terms from those documents and add them to the original query. Run the new query and present those results to the user. Typically converges quickly Based on Martin:

68 CSC 8520 Spring Paula Matuszek Blind Feedback Assume that first few documents returned are most relevant rather than having users identify them Proceed as for relevance feedback Tends to improve recall at the expense of precision Based on Martin:

69 CSC 8520 Spring Paula Matuszek Post-Hoc Analyses When a set of documents has been returned, they can be analyzed to improve usefulness in addressing information need –Grouped by meaning for polysemic queries (using N-Gram-type approaches) –Grouped by extracted information (Named entities, for instance) –Group into existing hierarchy if structured fields available –Filtering (e.g., eliminate spam)

70 CSC 8520 Spring Paula Matuszek Additional IR Issues In addition to improved relevance, can improve overall information retrieval with some other factors: –Eliminate duplicate documents –Provide good context –Use ontologies to provide synonym lists For the web: –Eliminate multiple documents from one site –Clearly identify paid links

71 CSC 8520 Spring Paula Matuszek Summary Information Retrieval is the process of returning documents to meet a user’s information need based on a query Typical methods are BOW (bag of words) which rely on keyword indexing with little semantic processing NLP techniques used including tokenizing, stemming, some parsing. Results can be improved by adding semantic information (such as thesauri) and by filtering and other post-hoc analyses.