INFO 624 Week 5 Text Properties and Operations

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INFO 624 -- Week 5Text Properties and Operations

• Dr. Xia Lin
• Assistant Professor
• College of Information Science and Technology
• Drexel University

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Objectives of Assignment 1

• Practice basic Web skills
• Get familiar with a few search engines
• Learn to describe features of search engines
• Learn to compare search engines

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Grading Sheet for Assignment 1

• Memo
• Selection of Search engines
• Is it downloadable
• Can it be under controlled of the small business?
  
• Quality of reviews
• Formats of review pages, including metadata.
• Appropriate links in reviews and in the
  registered page

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Whats missing?

• Who are the current users of the selected search
  engine?
• Hand-on experience with the selected search
  engines
• Personal observation or experience
• Some testing on demos or customer sites.
• Convincing statements on the differences between
  the search engines.

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Properties of Text

• Classic theories
• Zipfs Law
• Information Theory
• Benford's Law
• Bradford's Law
• Heaps Law
• English letter/word frequencies

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Zipfs Law (1945)

• in a large, well-written English document,
• r f c
• where
• r is the ranking number,
• f is the number of times the given word is
  used in the document c is a constant.
• Difference collections may have different c.
  English text tends to have c N/10 where N is
  the number of words in the collection.

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• Zipfs Law is an observation of a fact in
  proximity.
• Examples
• Word frequencies in Alice in Wonderland
• Time magazine collection
• Zipfs Law has been verified for many many years
  on many different collections.
• There are also many revised version of Ziphs Law.

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Example

• The word "the" is the most frequently occurring
  word in the novel "Moby Dick," occurring 1450
  times.
• The word "with" is the second-most frequently
  occurring word in that novel.
• How many times would we expect "with" to occur?
• How many times would we expect the third most
  frequently occurring word to appear?

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Information Theory

• Entropy (1948)
• Use the distribution of symbols to predict the
  amount of information in a text.
• Quantified measure for information
• Useful for (physical) data transfer
• And compression
• Not directly applicable to IR
• Example
• Which letter is likely to appear after a letter
  c is received?

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English Letter Usage Statistics

• Letter use frequencies
• E 72881 12.4
• T 52397 8.9
• A 47072 8.0
• O 45116 7.6
• N 41316 7.0
• I 39710 6.7
• H 38334 6.5

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• Doubled letter frequencies
• LL 2979 20.6
• EE 2146 14.8
• SS 2128 14.7
• OO 2064 14.3
• TT 1169 8.1
• RR 1068 7.4
• -- 701 4.8
• PP 628 4.3
• FF 430 2.9

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• Initial letter frequencies
• T 20665 15.2
• A 15564 11.4
• H 11623 8.5
• W 9597 7.0
• I 9468 6.9
• S 9376 6.9
• O 8205 6.0
• M 6293 4.6
• B 5831 4.2

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• Ending letter frequencies
• E 26439 19.4
• D 17313 12.7
• S 14737 10.8
• T 13685 10.0
• N 10525 7.7
• R 9491 6.9
• Y 7915 5.8
• O 6226 4.5

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Benford's Law

• If we randomly select a number from a table of
  statistical data, the probability that the first
  digit will be a "1" is about 0.301, rather than
  0.1 as we might expect if all digits were equally
  likely.

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Bradford's Law

• On a given subject, a few core journals will
  provide 1/3 of the articles on that subject, a
  medium number of secondary journals will provide
  another 1/3 of the articles on that subject, and
  a large number peripheral journals will provide
  the final 1/3 of the articles on that subject.

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For example

• If you found 300 citations for IR,
• 100 of those citations likely came from a core
  group of 5 journals,
• another 100 citations came from a group of 25
  journals,
• and the final 100 citations came from 125
  peripheral journals.
• Bradford expressed his law with this formula
  1nn

2
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Heaps Law

• The relationship of the size of vocabulary and
  the size of collections are V K n

b
Number of unique words
Text size
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Computerized Text Analysis

• Word (token) extraction
• Stop words
• Stemming
• Frequency counts
• Clustering

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Word Extraction

• Basic problems
• Digits
• Hyphens
• Punctuation
• Cases
• Lexical analyzer
• Define all possible characters into finite state
  machine
• Specify what states should cause the break of
  tokens.
• Example
• Parser.c

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Stop words

• Many of the most frequently used words in English
  are worthless in the indexing these words are
  called stop words.
• the, of, and, to, .
• Typically about 400 to 500 such words
• Why do we need to remove stop words?
• Reduce indexing file size
• stopwords accounts 20-30 of total word counts.
• Improve efficiency
• stop words are not useful for searching
• stop words always have a large number of hits

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Stop words

• Potential problems of removing stop words
• small stop list does not improve indexing much
• large stop list may eliminate some words that
  might be useful for someone or for some purposes
• stopwords might be part of phrases
• needs to process for both indexing and queries.
• Examples
• Lommoncommon.c
• commonwords

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Stemming

• Techniques used to find out the root/stem of a
  word
• lookup user engineering
• user 15 engineering 12
• users 4 engineered 23
• used 5 engineer
  12
• using 5
• stem use engineer

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Advantages of stemming

• improving effectiveness
• matching similar words
• reducing indexing size
• combing words with same roots may reduce indexing
  size as much as 40-50.
• Criteria for stemming
• correctness
• retrieval effectiveness
• compression performance

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Basic stemming methods

• Use tables and rules
• remove ending
• if a word ends with a consonant other than s,
• followed by an s, then delete s.
• if a word ends in es, drop the s.
• if a word ends in ing, delete the ing unless the
  remaining word consists only of one letter or of
  th.
• If a word ends with ed, preceded by a consonant,
  delete the ed unless this leaves only a single
  letter.
• ...

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• transform the remaining word
• if a word ends with ies but not eies or
  aies then ies --gt y.

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Example 1 Porter stem Algorithm

• A set of condition/action rules
• condition on the stem
• condition on the suffix
• condition on the rules
• different combination of conditions will activate
  different rules.
• Implementation
• stem.c
• Stem(word)
• ..
• ReplaceEnd(word, step1a_rule)
• ruleReplaceEnd(word, step1b_rule)
• if (rule106) (rule 107)
• ReplaceEnd(word, 1b1_rule)

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Example 2 Sound-based stemming

• Soundex rules
• letter Numeric equivalent
• B, F, P, V 1
• C, G, J, K, Q, S, X, Z 2
• D, T, 3
• L 4
• M, N, 5
• R, 6
• A, E, I, O, U, W, Y not coded
• Words sound similar often have same codes
• The code is not unique
• high compression rate

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Frequency counts

• The idea
• The best a computer can do is counting numbers
• counts the number of times a word occurred in a
  document
• counts the number of documents in a collection
  that contains a word
• Using occurrence frequencies to indicate relative
  importance of a word in a document
• if a word appears often in a document, the
  document likely deals with subjects related to
  the word.

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• Using occurrence frequencies to select most
  useful words to index a document collection
• if a word appears in every document, it is not a
  good indexing word
• if a word appears in only one or two documents,
  it may not be a good indexing word
• If a word appears in titles, each occurrence
  should be count 5(or 10) times.

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Automatic indexing

• 1. Parse individual words (tokens)
• 2. Remove stop words.
• 3. Stemming
• 4. Use frequency data
• decide heading threshold
• decide tail threshold
• decide variance of counting

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• 5. Create indexing structure
• invert indexing
• other structures

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Term Associations

• Counting word pairs
• If two words appear together very often, they are
  likely to be a phrase
• Counting document pairs
• if two documents have many common words, they are
  likely related

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More Counting

• Counting citation pairs
• If documents A and B both cite document C, D,
  then A and B might be related.
• If documents C and D often be cited together,
  they are likely related.
• Counting link patterns
• Get all pages that have links to my pages.
• Get all pages that contain similar links to my
  pages

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Google Search Engine

• Link analysis
• PageRank --The ranking of web pages are based on
  the number of links that refer to that web page
• If page A has a link to B, page A has one vote to
  B.
• The more votes a page get, the more useful the
  page is.
• If page A itself receives many votes, its vote to
  B will count more heavily
• Combining link analysis with word matching.

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ConceptLink

• Use terms co-occurring frequencies
• to predict semantic relationships
• to build concept clusters
• to suggest search terms
• Visualization of term relationships
• Link displays
• Map displays
• Drag-and drop interface for searching

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Document clustering

• Grouping similar documents to different sets
• Create similarity matrix
• Apply a hierarchical clustering algorithm
• 1 Identify the two closet documents and combine
  them into a cluster
• 2 Identify the next two closet documents and
  clusters and combine them into a clusters
• 3 If more then one cluster remains, return to
  step 1

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Application of Document Clustering

• Vivisimo
• Cluster search results on the fly
• Hierarchical categories for drill-down capability
  
• AltaVista
• Refine search
• Cluster related words into different groups based
  on their co-occurrence rates in documents.

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AltaVista
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Document Similarity

• Documents
• D1t11, t12, t13, , t1n
• D2t21, t22, t23, , t2n
• tik is either 0 or 1.
• Simple measurement of difference/ similarity
• wthe number of times t1k1, t2k1.
• xthe number of times t1k1, t2k0.
• ythe number of times t1k0, t2k1.
• zthe number of times t1k0, t2k0.

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Similarity Measure

• Cosine Coefficient
• The same as

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• D1s terms only n1wx (the number of times
  t1k1)
• D2s terms only n2wy (the number of times
  t2k1)
• Sameness count sc (wz)/(n1n2)
• Difference count dc (xy)/(n1n2)
• Rectangular Distance rd MAX(n1, n2)
• Conditional probability cpmin(n1, n2)
• mean mean (n1n2)/2

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Similarity Measure

• Dices Coefficient
• Dice(D1, D2) 2w/(n1n2)
• where w is the number of terms that D1, and D2
  have in common n1, n2 are the number of terms
  in D1and D2.
• Jaccard Coefficient
• Jaccard(D1, D2) w/(N-z)
• w/(n1n2-w)

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Similarity Metric

• A metric has three defining properties
• Its value are non-negative
• Its symmetric
• It satisfies the triangle inequality
• AC?ABBC

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Lp Metrics
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Similarity Matrix

• Pairwise coupling of similarities among a group
  of documents
• S11 S12 S13 S14 S15 S16 S17 S18
• S21 S22 S23 S24 S25 S26 S27 S28
• S31 S32 S33 S34 S35 S36 S37 S38
• S41 S42 S43 S44 S45 S46 S47 S48
• S51 S52 S53 S54 S55 S56 S57 S58
• S61 S62 S63 S64 S65 S66 S67 S68
• S71 S72 S73 S74 S75 S76 S77 S78
• S81 S82 S83 S84 S85 S86 S87 S88

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MetaData

• Data about data
• Descriptive Data
• External to the meaning of the document
• Dublin Core Metadata Element Set
• Author, title, publisher, etc.
• Semantic Metadata
• Subject indexing
• Challenge automatic generation of metadata for
  documents

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Markup Languages
SGML
XSL
XML
HyTime
Metalanguage Languages
HTML
CSS
RDF
MathML
SMIL
Semantic Web?
Stylesheet
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Midterm

• Concepts
• What is information retrieval?
• Data, information, text, and documents
• Two abstractions principles
• Users information needs
• Queries and query formats
• Precision and Recall
• Relevance
• Zipfs Law, Benford's Law

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Midterm

• Procedures problem solving
• How to translate a request into a query?
• How to expand queries
• for better recall or better precision?
• How to create an inverted indexing?
• How to create a vector space ?
• How to calculate similarities of documents?
• How to match a query to documents in a vector
  space?

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• Discussions
• Challenges of IR
• Advantages and disadvantages of Boolean search
  (vector space, automatic indexing,
  association-based queries, etc.)
• Evaluation of IR systems
• With or without using precision/recall.
• Difference between data retrieval and information
  retrieval