2010.02.01- SLIDE 1IS 240 – Spring 2010

Prof. Ray Larson University of California, Berkeley

School of Information

Principles of Information Retrieval

Lecture 4: IR System Elements (cont)

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Review• Review

– Elements of IR Systems• Collections, Queries• Text processing and Zipf distribution

• Stemmers and Morphological analysis (cont…)

• Inverted file indexes

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Queries• A query is some expression of a user’s

information needs• Can take many forms

– Natural language description of need– Formal query in a query language

• Queries may not be accurate expressions of the information need– Differences between conversation with a

person and formal query expression

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Collections of Documents…• Documents

– A document is a representation of some aggregation of information, treated as a unit.

• Collection– A collection is some physical or logical

aggregation of documents• Let’s take the simplest case, and say we

are dealing with a computer file of plain ASCII text, where each line represents the “UNIT” or document.

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How to search that collection?• Manually?

– Cat, more• Scan for strings?

– Grep• Extract individual words to search???

– “tokenize” (a unix pipeline)• tr -sc ’A-Za-z’ ’\012’ < TEXTFILE | sort | uniq –c

– See “Unix for Poets” by Ken Church

• Put it in a DBMS and use pattern matching there…– assuming the lines are smaller than the text size limits

for the DBMS

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What about VERY big files?• Scanning becomes a problem• The nature of the problem starts to change

as the scale of the collection increases• A variant of Parkinson’s Law that applies

to databases is:– Data expands to fill the space available to

store it

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Document Processing Steps

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Structure of an IR SystemSearchLine Interest profiles

& QueriesDocuments

& data

Rules of the game =Rules for subject indexing +

Thesaurus (which consists of

Lead-InVocabulary

andIndexing

Language

StorageLine

Potentially Relevant

Documents

Comparison/Matching

Store1: Profiles/Search requests

Store2: Documentrepresentations

Indexing (Descriptive and

Subject)

Formulating query in terms of

descriptors

Storage of profiles Storage of

Documents

Information Storage and Retrieval System

Adapted from Soergel, p. 19

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Query Processing• In order to correctly match queries and

documents they must go through the same text processing steps as the documents did when they were stored

• In effect, the query is treated like it was a document

• Exceptions (of course) include things like structured query languages that must be parsed to extract the search terms and requested operations from the query– The search terms must still go through the same text

process steps as the document…

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Steps in Query processing• Parsing and analysis of the query text

(same as done for the document text)– Morphological Analysis– Statistical Analysis of text

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Stemming and Morphological Analysis

• Goal: “normalize” similar words• Morphology (“form” of words)

– Inflectional Morphology• E.g,. inflect verb endings and noun number• Never change grammatical class

– dog, dogs– tengo, tienes, tiene, tenemos, tienen

– Derivational Morphology • Derive one word from another, • Often change grammatical class

– build, building; health, healthy

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Plotting Word Frequency by Rank

• Say for a text with 100 tokens• Count

– How many tokens occur 1 time (50)– How many tokens occur 2 times (20) …– How many tokens occur 7 times (10) … – How many tokens occur 12 times (1)– How many tokens occur 14 times (1)

• So things that occur the most often share the highest rank (rank 1).

• Things that occur the fewest times have the lowest rank (rank n).

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Many similar distributions…• Words in a text collection• Library book checkout patterns• Bradford’s and Lotka’s laws.• Incoming Web Page Requests (Nielsen)• Outgoing Web Page Requests (Cunha &

Crovella)• Document Size on Web (Cunha &

Crovella)

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Zipf Distribution(linear and log scale)

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Resolving Power (van Rijsbergen 79)

The most frequent words are not the most descriptive.

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Other Models• Poisson distribution• 2-Poisson Model• Negative Binomial• Katz K-mixture

– See Church (SIGIR 1995)

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Stemming and Morphological Analysis

• Goal: “normalize” similar words• Morphology (“form” of words)

– Inflectional Morphology• E.g,. inflect verb endings and noun number• Never change grammatical class

– dog, dogs– tengo, tienes, tiene, tenemos, tienen

– Derivational Morphology • Derive one word from another, • Often change grammatical class

– build, building; health, healthy

2010.02.01- SLIDE 19IS 240 – Spring 2010

Stemming and Morphological Analysis• Goal: “normalize” similar words• Morphology (“form” of words)

– Inflectional Morphology• E.g,. inflect verb endings and noun number• Never change grammatical class

– dog, dogs– tengo, tienes, tiene, tenemos, tienen

– Derivational Morphology • Derive one word from another, • Often change grammatical class

– build, building; health, healthy

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Simple “S” stemming• IF a word ends in “ies”, but not “eies” or

“aies”– THEN “ies” “y”

• IF a word ends in “es”, but not “aes”, “ees”, or “oes”– THEN “es” “e”

• IF a word ends in “s”, but not “us” or “ss”– THEN “s” NULL

Harman, JASIS Jan. 1991

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Stemmer ExamplesThe SMART

stemmerThe Porterstemmer

The IAGO!stemmer

% tstem ateate% tstem applesappl% tstem formulaeformul% tstem appendicesappendix% tstem implementationimple% tstem glassesglass

% pstem ateat% pstem applesappl% pstem formulaeformula% pstem appendicesappendic% pstem implementationimplement% pstem glassesglass

% stemate|2eat|2apples|1apple|1formulae|1formula|1appendices|1appendix|1implementation|1implementation|1glasses|1 glasses|1

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Too Aggressive Too Timid

organization/organpolicy/police

execute/executivearm/army

european/europecylinder/cylindrical

create/creationsearch/searcher

Errors Generated by Porter Stemmer (Krovetz 93)

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Automated Methods• Stemmers:

– Very dumb rules work well (for English)– Porter Stemmer: Iteratively remove suffixes– Improvement: pass results through a lexicon

• Newer stemmers are configurable (Snowball)– Demo…

• Powerful multilingual tools exist for morphological analysis– PCKimmo, Xerox Lexical technology– Require a grammar and dictionary– Use “two-level” automata– Wordnet “morpher”

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Wordnet• Type “wn word” on a machine where

wordnet is installed…• Large exception dictionary:• Demo

aardwolves aardwolf abaci abacus abacuses abacus abbacies abbacy abhenries abhenry abilities ability abkhaz abkhaz abnormalities abnormality aboideaus aboideau aboideaux aboideau aboiteaus aboiteau aboiteaux aboiteau abos abo abscissae abscissa abscissas abscissa absurdities absurdity…

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Using NLP• Strzalkowski (in Reader)

Text NLP repres Dbasesearch

TAGGERNLP: PARSER TERMS

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Using NLP

INPUT SENTENCEThe former Soviet President has been a local hero ever sincea Russian tank invaded Wisconsin.

TAGGED SENTENCEThe/dt former/jj Soviet/jj President/nn has/vbz been/vbn a/dt local/jj hero/nn ever/rb since/in a/dt Russian/jj tank/nn invaded/vbd Wisconsin/np ./per

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Using NLP

TAGGED & STEMMED SENTENCEthe/dt former/jj soviet/jj president/nn have/vbz be/vbn a/dt local/jj hero/nn ever/rb since/in a/dt russian/jj tank/nn invade/vbd wisconsin/np ./per

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Using NLP

PARSED SENTENCE[assert [[perf [have]][[verb[BE]] [subject [np[n PRESIDENT][t_pos THE] [adj[FORMER]][adj[SOVIET]]]] [adv EVER] [sub_ord[SINCE [[verb[INVADE]] [subject [np [n TANK][t_pos A] [adj [RUSSIAN]]]] [object [np [name [WISCONSIN]]]]]]]]]

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Using NLP

EXTRACTED TERMS & WEIGHTSPresident 2.623519 soviet 5.416102President+soviet 11.556747 president+former 14.594883Hero 7.896426 hero+local 14.314775Invade 8.435012 tank 6.848128Tank+invade 17.402237 tank+russian 16.030809Russian 7.383342 wisconsin 7.785689

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Same Sentence, different sysEnju ParserROOT ROOT ROOT ROOT -1 ROOT been be VBN VB 5been be VBN VB 5 ARG1 President president NNP NNP 3been be VBN VB 5 ARG2 hero hero NN NN 8a a DT DT 6 ARG1 hero hero NN NN 8a a DT DT 11 ARG1 tank tank NN NN 13local local JJ JJ 7 ARG1 hero hero NN NN 8The the DT DT 0 ARG1 President president NNP NNP 3former former JJ JJ 1 ARG1 President president NNP NNP 3Russian russian JJ JJ 12 ARG1 tank tank NN NN 13Soviet soviet NNP NNP 2 MOD President president NNP NNP 3invaded invade VBD VB 14 ARG1 tank tank NN NN 13invaded invade VBD VB 14 ARG2 Wisconsin wisconsin NNP NNP 15has have VBZ VB 4 ARG1 President president NNP NNP 3has have VBZ VB 4 ARG2 been be VBN VB 5since since IN IN 10 MOD been be VBN VB 5since since IN IN 10 ARG1 invaded invade VBD VB 14ever ever RB RB 9 ARG1 since since IN IN 10

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Other Considerations• Church (SIGIR 1995) looked at

correlations between forms of words in texts

hostages nullhostage 619(a) 479(b)null 648(c) 78223(d)

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Assumptions in IR• Statistical independence of terms• Dependence approximations

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Statistical Independence Two events x and y are statistically

independent if the product of their probability of their happening individually equals their probability of happening together.

),()()( yxPyPxP =

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Statistical Independence and Dependence• What are examples of things that are

statistically independent?

• What are examples of things that are statistically dependent?

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Statistical Independence vs. Statistical Dependence• How likely is a red car to drive by given we’ve

seen a black one?

• How likely is the word “ambulence” to appear, given that we’ve seen “car accident”?

• Color of cars driving by are independent (although more frequent colors are more likely)

• Words in text are not independent (although again more frequent words are more likely)

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Lexical Associations• Subjects write first word that comes to mind

– doctor/nurse; black/white (Palermo & Jenkins 64)

• Text Corpora yield similar associations• One measure: Mutual Information (Church and Hanks

89)

• If word occurrences were independent, the numerator and denominator would be equal (if measured across a large collection)

)(),(),(log),( 2 yPxPyxPyxI =

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Interesting Associations with “Doctor”

(AP Corpus, N=15 million, Church & Hanks 89)

I(x,y) f(x,y) f(x) x f(y) y11.311.310.79.49.08.98.7

12830861125

1111105110511052751105621

honorarydoctorsdoctorsdoctorsexamineddoctorsdoctor

621442411546213171407

doctordentistsnursestreatingdoctortreatbills

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These associations were likely to happen because the non-doctor words shown here are very commonand therefore likely to co-occur with any noun.

Un-Interesting Associations with “Doctor”

I(x,y) f(x,y) f(x) x f(y) y0.960.950.93

64112

62128469084716

doctorais

7378511051105

withdoctorsdoctors

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Query Processing• Once the text is in a form to match to the

indexes then the fun begins– What approach to use?

• Boolean?• Extended Boolean?• Ranked

– Fuzzy sets?– Vector?– Probabilistic?– Language Models? – Neural nets?

• Most of the next few weeks will be looking at these different approaches

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Display and formatting• Have to present the the results to the user• Lots of different options here, mostly

governed by – How the actual document is stored – And whether the full document or just the

metadata about it is presented

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What to do with terms…• Once terms have been extracted from the

documents, they need to be stored in some way that lets you get back to documents that those terms came from

• The most common index structure to do this in IR systems is the “Inverted File”

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Boolean Implementation: Inverted Files

• We will look at “Vector files” in detail later. But conceptually, an Inverted File is a vector file “inverted” so that rows become columns and columns become rows

docs t1 t2 t3D1 1 0 1D2 1 0 0D3 0 1 1D4 1 0 0D5 1 1 1D6 1 1 0D7 0 1 0D8 0 1 0D9 0 0 1

D10 0 1 1

Terms D1 D2 D3 D4 D5 D6 D7 …t1 1 1 0 1 1 1 0t2 0 0 1 0 1 1 1t3 1 0 1 0 1 0 0

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How Are Inverted Files Created

• Documents are parsed to extract words (or stems) and these are saved with the Document ID.

Now is the timefor all good men

to come to the aidof their country

Doc 1

It was a dark andstormy night in

the country manor. The time was past midnight

Doc 2

Term Doc #now 1is 1the 1time 1for 1all 1good 1men 1to 1come 1to 1the 1aid 1of 1their 1country 1it 2was 2a 2dark 2and 2stormy 2night 2in 2the 2country 2manor 2the 2time 2was 2past 2midnight 2

TextProcSteps

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How Inverted Files are Created

• After all document have been parsed the inverted file is sorted

Term Doc #a 2aid 1all 1and 2come 1country 1country 2dark 2for 1good 1in 2is 1it 2manor 2men 1midnight 2night 2now 1of 1past 2stormy 2the 1the 1the 2the 2their 1time 1time 2to 1to 1was 2was 2

Term Doc #now 1is 1the 1time 1for 1all 1good 1men 1to 1come 1to 1the 1aid 1of 1their 1country 1it 2was 2a 2dark 2and 2stormy 2night 2in 2the 2country 2manor 2the 2time 2was 2past 2midnight 2

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How Inverted Files are Created

• Multiple term entries for a single document are merged and frequency information added

Term Doc # Freqa 2 1aid 1 1all 1 1and 2 1come 1 1country 1 1country 2 1dark 2 1for 1 1good 1 1in 2 1is 1 1it 2 1manor 2 1men 1 1midnight 2 1night 2 1now 1 1of 1 1past 2 1stormy 2 1the 1 2the 2 2their 1 1time 1 1time 2 1to 1 2was 2 2

Term Doc #a 2aid 1all 1and 2come 1country 1country 2dark 2for 1good 1in 2is 1it 2manor 2men 1midnight 2night 2now 1of 1past 2stormy 2the 1the 1the 2the 2their 1time 1time 2to 1to 1was 2was 2

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Inverted Files• The file is commonly split into a Dictionary

and a Postings fileTerm Doc # Freqa 2 1aid 1 1all 1 1and 2 1come 1 1country 1 1country 2 1dark 2 1for 1 1good 1 1in 2 1is 1 1it 2 1manor 2 1men 1 1midnight 2 1night 2 1now 1 1of 1 1past 2 1stormy 2 1the 1 2the 2 2their 1 1time 1 1time 2 1to 1 2was 2 2

Doc # Freq2 11 11 12 11 11 12 12 11 11 12 11 12 12 11 12 12 11 11 12 12 11 22 21 11 12 11 22 2

Term N docs Tot Freqa 1 1aid 1 1all 1 1and 1 1come 1 1country 2 2dark 1 1for 1 1good 1 1in 1 1is 1 1it 1 1manor 1 1men 1 1midnight 1 1night 1 1now 1 1of 1 1past 1 1stormy 1 1the 2 4their 1 1time 2 2to 1 2was 1 2

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Inverted files• Permit fast search for individual terms• Search results for each term is a list of

document IDs (and optionally, frequency and/or positional information)

• These lists can be used to solve Boolean queries:– country: d1, d2– manor: d2– country and manor: d2

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Inverted Files• Lots of alternative implementations

– E.g.: Cheshire builds within-document frequency using a hash table during document parsing. Then Document IDs and frequency info are stored in a BerkeleyDB B-tree index keyed by the term.

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Btree (conceptual)

B | | D | | F |

AcesBoilers

Cars

F | | P | | Z |

R | | S | | Z |H | | L | | P |

DevilsMinors

PanthersSeminoles

FlyersHawkeyesHoosiers

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Btree with Postings

B | | D | | F |

AcesBoilers

Cars

F | | P | | Z |

R | | S | | Z |H | | L | | P |

DevilsMinors

PanthersSeminoles

FlyersHawkeyesHoosiers

2,4,8,122,4,8,122,4,8,12

2,4,8,122,4,8,12

2,4,8,125, 7, 200

2,4,8,122,4,8,128,120

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Inverted files• Permit fast search for individual terms• Search results for each term is a list of

document IDs (and optionally, frequency, part of speech and/or positional information)

• These lists can be used to solve Boolean queries:– country: d1, d2– manor: d2– country and manor: d2

2010.02.01- SLIDE 52IS 240 – Spring 2010

Query Processing• Once the text is in a form to match to the

indexes then the fun begins– What approach to use?

• Boolean?• Extended Boolean?• Ranked

– Fuzzy sets?– Vector?– Probabilistic?– Language Models? – Neural nets?

• Most of the next few weeks will be looking at these different approaches

2010.02.01- SLIDE 53IS 240 – Spring 2010

Display and formatting• Have to present the the results to the user• Lots of different options here, mostly

governed by – How the actual document is stored – And whether the full document or just the

metadata about it is presented