This lecture. Introduction to information retrieval. Making money with information retrieval. Some technical basics. Link analysis.

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2 This lecture Introduction to information retrieval. Making money with information retrieval. Some technical basics. Link analysis. CSC401/2511 Spring

3 Information retrieval systems Information retrieval (IR): n. searching for documents or information in documents. Question-answering: respond with a specific answer to a question (e.g., Wolfram Alpha). Document retrieval: find documents relevant to a query, ranked by relevance (e.g., bing or Google). Text analytics/data mining: General organization of large textual databases (e.g., Lexis-Nexis, OpenText, MedSearch,.) CSC401/2511 Spring

4 Terminology Information retrieval has slightly different terminology than the tasks we ve seen previously: Document: a book, article, web page, or paragraph Collection: Term: Stop word: (depending on the task and data). a corpus of documents a word type a functional (non-content) word (e.g., the) CSC401/2511 Spring

5 Query types Different kinds of questions can be asked. Factoid questions, e.g., How often were the peace talks in Ireland delayed or disrupted as a result of acts of violence? Narrative (open-ended) questions, e.g., Can you tell me about contemporary interest in the Greek philosophy of stoicism? Complex/hybrid questions, e.g., Who was involved in the Schengen agreement to eliminate border controls in Western Europe and what did they hope to accomplish? CSC401/2511 Spring

6 Question answering (QA) Which woman has won more than 1 Nobel prize? (Marie Curie) Question Answering (QA) usually involves a specific answer to a question. CSC401/2511 Spring

7 Document retrieval vs IR One strategy is to turn question answering into information retrieval (IR) and let the human complete the task. CSC401/2511 Spring

8 Question answering (QA) CSC401/2511 Spring

9 Knowledge-based QA 1. Build a structured semantic representation of the query. Extract times, dates, locations, entities using regular expressions. Fit to well-known templates. CSC401/2511 Spring Query databases with these semantics. Ontologies (Wikipedia infoboxes). Restaurant review databases. Calendars. Movie schedules.

10 IR-based QA CSC401/2511 Spring

11 IR-based QA Information retrieval Question answering CSC401/2511 Spring

12 IBM s Watson Human 1 Game Control System Clue Grid Decisions to Buzz and Bet Strategy Watson s Game Controller Text-to-Speech Clue & Category Answers & Confidences Watson s QA Engine 2,880 IBM Power750 Compute Cores 15 TB of Memory Human 2 Clues, Scores & Other Game Data Content equivalent to ~ 1,000,000 books source: A Brief Overview and Thoughts for Healthcare Education and Performance Improvement by the IBM Watson team CSC401/2511 Spring

13 IBM s Watson: search This man became the 44 th President of the United States in 2008 CSC401/2511 Spring

14 IBM s Watson: search Title-oriented search: In some cases, the solution is in the title of highly-ranked documents. E.g., This pizza delivery boy celebrated New Year s at Applied Cryogenics. CSC401/2511 Spring

15 IBM s Watson: selection Once candidates have been gathered from various sources and methods, rank them according to various scores (IBM Watson uses >50 scoring metrics). In cell division, mitosis splits the nucleus & cytokinesis splits this liquid cushioning the nucleus CSC401/2511 Spring

16 IBM s Watson: selection One aspect of Jeopardy! is that answers are often posed with puns that have to be disambiguated. Bilbo shouldn t have played riddles in the dark with this shady character from WordNet s Synonym-sets CSC401/2511 Spring

17 How to make money out of this? CSC401/2511 Spring

18 Making money before search Advertisers used to pay for banner ads that did not depend on user queries. CPM (Cost per mille): Pay for each ad display. CPC (Cost per click): Pay when user clicks an ad. CTR (Click through rate): Fraction of ad displays that result in click-throughs. CPA (Cost per action): Pay only when user makes online purchase after click-through. CSC401/2511 Spring

19 Making money with search Advertisers now bid for keywords. Ads are displayed for the highest bidders when a query contains those keywords. PPC (Pay per click): CPC for ads served based on a ranking of bid keywords and user interest (e.g., Google AdWords). (it s a bit more complicated ) CSC401/2511 Spring

20 How are ads ranked? Today, a two-bid process is typical. First, organizations bid on keywords By itself, this can lead to abuse, monopolization, and irrelevant content. Second, we re-rank based on relevance based on click-through. CSC401/2511 Spring

21 How are ads ranked? Advertiser Bid CTR Ad rank Rank Paid A $ (minimum) B $ $2.68 C $ $1.51 D $ $0.51 Bid: amount determined by advertiser for keyword. CTR: click-through rate an approximation of relevance. Ad rank: Bid CTR trades off advertiser and user interests. Rank: actual rank. Paid: Minimum amount necessary to maintain rank + 1. CSC401/2511 Spring

22 How are ads ranked? Advertiser Bid CTR Ad rank Rank Paid A $ (minimum) B $ $2.68 C $ $1.51 D $ $0.51 Paid: Minimum amount necessary to maintain rank + 1. Paid r CTR r = Bid r+1 CTR r+1 + $0.01 Paid r = Bid r+1 CTR r+1 CTR r + $0.01 E.g., Paid 1 = $ $0.01 = $1.51 CSC401/2511 Spring

23 Aside highest paying search terms (according to $69.10 mesothelioma treatment options $66.46 mesothelioma risk $65.85 personal injury lawyer michigan $65.74 michigan personal injury attorney $62.59 student loans consolidation $61.44 car accident attorney los angeles $61.26 mesothelioma survival rate $60.96 treatment of mesothelioma $59.44 online car insurance quotes $59.39 arizona dui lawyer CSC401/2511 Spring

24 Back to basics. How do we find the right documents for a query? CSC401/2511 Spring

25 Queries A query is a textual key which orders a specific subset of documents (or answers) in a collection. Historically, these were highly structured in a logical language, but in modern search engines queries are more often streams of syntactically disconnected keywords. A boolean query is a logical combination of boolean membership predicates. Brutus AND Caesar AND NOT Calpurnia CSC401/2511 Spring

26 Term-document incidence Anthony and Cleopatra Julius Caesar The Tempest Hamlet Othello Macbeth ANTHONY BRUTUS CAESAR CALPURNIA CLEOPATRA MERCY WORSER For the query Brutus AND Caesar AND NOT Calpurnia, (Brutus) (Caesar) (Not Calpurnia) (Bitwise AND) CSC401/2511 Spring

27 Boolean Queries and big collections If we have 1 million documents, each with 1000 tokens 1 billion tokens at most 1 billion 1 s in the matrix. If we have 500,000 distinct terms, the term-document incidence matrix will have 500,000,000,000 elements. There will be << 1 billion 1s in this matrix. Very sparse and a waste of space. Can there be a better way? CSC401/2511 Spring

28 Inverted index Given a query word, the inverted index for that word gives us all documents that contain that word in either the title, the abstract (summary), some hidden metadata, or the entire text. More sophisticated versions also include the frequency and positions of the query word in each document. Matlab query Inverted index D 1 documents How does one construct such indices? CSC401/2511 Spring

29 Inverted index construction 1. Collect the documents to be indexed. Friends, Romans, countrymen So let it be with Caesar 2. Tokenize the text. Friends Romans countrymen So 3. Do preprocessing and normalization, resulting in the indexing terms. friend roman countryman so 4. Create a dictionary (hash) of documents given terms. CSC401/2511 Spring

30 Simple conjunctive query Given the query Brutus AND Calpurnia, 1. Locate Brutus in the dictionary. Retrieve documents list. 2. Locate Calpurnia in the dictionary. Retrieve documents list. 3. Intersect the two document lists. Return the result to the user. Linear in the lengths of document lists. (if lists are sorted) CSC401/2511 Spring

31 Constructing indices Spiders (aka. Robots, bots, crawlers) start with root (seed) URLs. Follow all links on these pages recursively. Novel pages are processed and indexed. Despite the exponential growth in memory across depth, breadth-first search is quite popular. Depth-first search is linear in depth, but can get lost. Trivia: If you click on the first contentful link in any Wikipedia page, you will eventually be led to the Philosophy article. CSC401/2511 Spring

32 Increasing entropy? Boolean retrieval is precise and was very popular for decades (it still is used for structured data, like desktop file search). The amount and value of unstructured data (i.e., text) has grown faster than structured data on the web Unstructured Structured Data volume Market cap (data from Chris Manning) Data volume Market cap CSC401/2511 Spring

33 Zipf s law on the web These variables have Zipfian distributions: Number of links to and links from a page. Length of web pages. Number of web page hits. (graph from Ray Mooney) CSC401/2511 Spring

34 New challenges for IR on the web Distributed data: Documents spread over millions of web servers. Volatile data: Document change or disappear frequently and rapidly. Large volume: Petabytes of data. Poor quality: No editorial control, false information, poor writing, typographic errours. Heterogeneity: Various media, languages, encodings. Unstructured: No uniform structure, HTML errors, CSC401/2511 Spring duplicate documents.

35 Detecting duplicates duplicates The user will become annoyed when many top-ranking hits are identical/similar. Nearly-identical pages can be determined by hashing E.g., don t index en.m.wikipedia.org/wiki/ if you ve indexed en.wikipedia.org/wiki/. Zero marginal relevance occurs when a highly relevant document becomes irrelevant by being ranked below a (near-)duplicate. CSC401/2511 Spring

36 Detecting duplicates duplicates Compute similarity with some edit-distance measure. Syntactic similarity (e.g., overlap of bigrams) easier to measure than semantic similarity. If this measure is above some threshold θ for some pair of documents, we consider them duplicates. Jaccard coefficient: J A, B = A B A B Is a measure of similarity on [0.. 1] J A, A = 1 J A, B = 0 iff A B = CSC401/2511 Spring

37 Jaccard coefficient on 2-grams Documents: d 1 : Jack London went to Toronto d 2 : Jack London went to the city of Toronto d 3 : Jack went from Toronto to London J d 1, d 2 = 3 8 = J d 1, d 3 = 0 CSC401/2511 Spring

38 Link analysis When we re crawling the web and indexing, we want to retain some record of similarity between (non-duplicate) documents in terms of their link structure. This will help in searching. CSC401/2511 Spring

39 Bibliometrics: citation analysis Impact factor: Developed in 1972 to measure the quality and influence of scientific journals. Measures how often articles are cited. Bibliographic coupling: Measure of similarity between documents according to the intersection of their citations (Kessler, 1963). A B CSC401/2511 Spring

40 Bibliometrics: citation analysis Co-citation: Measure of similarity between documents according to the intersection of the documents that cite them (Small, 1973). A B CSC401/2511 Spring

41 Links are not citations Many links are navigational within a website. Many pages with high in-degree are portals without much content. Some links are not necessarily endorsements. Relevance of citations in scientific settings is (theoretically) enforced by peer review. Can we mimic the enforcement of relevance usually done by human experts in scientific articles? CSC401/2511 Spring

42 Authorities and hubs Authorities are pages recognized as significant, trustworthy, and useful for a topic. In-degree (number of incoming links) is an estimate of authority. Should incoming links from authoritative pages count more than others? Hubs are index pages that provide lots of links to relevant content pages. e.g., reddit.com is a hub page for recycled memes. CSC401/2511 Spring

43 HITS The HITS algorithm (Kleinberg, 1998) attempts to learn hubs and authorities on a given topic given relevant web subgraphs. Hubs and authorities tend to form bipartite graphs. Hubs Authorities CSC401/2511 Spring

44 HITS First, find (top N) most relevant pages for a query this is the root set, R. (we ll see how to do this next lecture) Next, look at the link structure relative to R. The base set, S is R and all pages that link to and are linked from pages in R S R CSC401/2511 Spring

45 HITS: Authorities and In-degree Even for S, nodes with high in-degree may not be authorities they may just be generically popular pages. Authority should be determined by strong hubs. Iteratively (slowly) converge on a mutually reinforcing set of hubs and authorities. For every page p S, maintain Authority score: a p (initialized to 1/ S ) Hub score: h p (initialized to 1/ S ) subject to p S a 2 p = 1 = 2 p S h p CSC401/2511 Spring

46 HITS update rules Authorities p are pointed to ( ) by lots of good hubs q: a p = q:q p h q a 4 = h 1 + h 2 + h 3 Hubs point to lots of good authorities: h q = a p p:q p h 4 = a 1 + a 2 + a 3 CSC401/2511 Spring

47 Page similarity using HITS Given honda.com, we also get: toyota.com ford.com bmwusa.com saturn.com nissanmotors.com This method can have trouble with ambiguous queries, however CSC401/2511 Spring

48 PageRank PageRank (Brin & Page, 1998) is an alternative to HITS that does not distinguish between hub and authority. CSC401/2511 Spring

49 PageRank initial idea Assume that in-degree does not account for the authority of the source of a link. For page p, the page rank is: where R p = c CSC401/2511 Spring q:q p R(q) N q N q is the total number of out-links over all q. c is a normalizing constant. A page s rank flows out equally among outgoing links.

50 PageRank flow of authority PageRank would iteratively adjust all R p until overall page ranking converged Steady state CSC401/2511 Spring

51 PageRank problem Groups of purely self-referential pages (linked from the outside) are sinks that absorb all the rank in the system during the iterative rank assignment process. CSC401/2511 Spring

52 PageRank rank source An ethereal rank source E continually replenishes the rank of each page p by a fixed amount E p R p = c q:q p R(q) N q + E(p) CSC401/2511 Spring

53 Complete ranking A complete ranking involves combining: PageRank. Preferences using HTML tags (e.g., title or abstract are often highly informative). Similarity of query words and documents. How do we relate query words and documents in the first place? CSC401/2511 Spring

54 Next lecture How to relate query terms and documents. Vector space model. How to generalize query terms. Latent semantic indexing. How to rank documents. Singular value decomposition. How to evaluate different search engines. CSC401/2511 Spring

55 Misc Some slide and material based on those of Ray J. Mooney (UTexas, CS371R), Hinrich Schütze, Christina Lioma, and Chris Manning (Stanford, CS276). Dan Jurafsky (Stanford, CS124) CSC401/2511 Spring

56 Aside PageRank algorithm Given the total set of pages S, Let p S: E p = α for some 0 α 1 S Initialize p S: R p = 1/ S Until convergence: For each p S: R R q p 1 α + E(p) q:q p N q 1 c p S R p For each p S: R p cr (p) //normalize CSC401/2511 Spring