the boolean model simple model based on set theory queries specified as boolean expressions...

The Boolean Model

•Simple model based on set theory

•Queries specified as boolean expressions – precise semantics– neat formalism– q = ka (kb kc)

•Terms are either present or absent. Thus, wij {0,1}

•Consider– q = ka (kb kc)– vec(qdnf) = (1,1,1) (1,1,0) (1,0,0)– vec(qcc) = (1,1,0) is a conjunctive component

•Each query can be transformed in DNF form

The Boolean Model

•q = ka (kb kc)

•sim(q,dj) = 1, if document satisfies the boolean query

0 otherwise

- no in-between, only 0 or 1

(1,1,1)

(1,0,0)(1,1,0)

Ka Kb

Kc

Exercise

D1 = “computer information retrieval”

D2 = “computer retrieval”

D3 = “information”

D4 = “computer information”

Q1 = “information retrieval”

Q2 = “information ¬computer”

Exercise

กำ��หนด Index term ของแต่ ละเอกำส�รD1 = {love, need, person, possess, understand}D2 = {heart, listen, love, practice, suffer}D3 = {compassion, love, mind, person, practice}D4 = {death, health, languor, life, suffer}D5 = {energy, love, nourish, practice, teach}

Q = {love ^ suffer}

Drawbacks of the Boolean Model

• Retrieval based on binary decision criteria with no notion of partial matching

• No ranking of the documents is provided (absence of a grading scale)

• Information need has to be translated into a Boolean expression which most users find awkward

• The Boolean queries formulated by the users are most often too simplistic

• As a consequence, the Boolean model frequently returns either too few or too many documents in response to a user query

Drawbacks of the Boolean Model

The Boolean model imposes a binary criterion for deciding relevance

The question of how to extend the Boolean model to accomodate partial matching and a ranking has attracted considerable attention in the past

Two extensions of boolean model:– Fuzzy Set Model

– Extended Boolean Model

Set Theoretic Models

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IR Models

Non-Overlapping ListsProximal Nodes

Structured Models

Retrieval: Adhoc

Filtering

Browsing

U s e r

T a s k

Classic Models

Boolean Vector

Probabilistic

Set Theoretic

Fuzzy Extended Boolean

Probabilistic

Inference Network Belief Network

Algebraic

Generalized Vector Lat. Semantic Index

Neural Networks

Browsing

Flat Structure Guided

Hypertext 8

Set Theoretic Models

The Boolean model imposes a binary criterion for deciding relevance

The question of how to extend the Boolean model to accomodate partial matching and a ranking has attracted considerable attention in the past

Two set theoretic models for this: Fuzzy Set Model Extended Boolean Model

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Fuzzy Set Model

Queries and docs represented by sets of index terms: matching is approximate from the start

This vagueness can be modeled using a fuzzy framework, as follows:with each term is associated a fuzzy seteach doc has a degree of membership in this fuzzy

set This interpretation provides the foundation for many

models for IR based on fuzzy theory In here, we discuss the model proposed by Ogawa,

Morita, and Kobayashi (1991)10

Fuzzy Set Theory

Framework for representing classes whose boundaries are not well defined

Key idea is to introduce the notion of a degree of membership associated with the elements of a set

This degree of membership varies from 0 to 1 and allows modeling the notion of marginal membership

Thus, membership is now a gradual notion, contrary to the crispy notion enforced by classic Boolean logic

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12

Fuzzy Set Theory

Model A query term: a fuzzy set A document: degree of membership in this test Membership function

Associate membership function with the elements of the class

0: no membership in the test 1: full membership 0 ~1: marginal elements of the test

documents

Fuzzy Set Theory

A fuzzy subset A of a universe of discourse U is characterized by a membership function µA: U[0,1] which associates with each element u of U a number µA(u) in the interval [0,1]

– complement:

– union:

– intersection:

)(1)( uu AA

))(),(max()( uuu BABA

))(),(min()( uuu BABA

a class document collectionfor query term

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Examples

Assume U={d1, d2, d3, d4, d5, d6} Let A and B be {d1, d2, d3} and {d2, d3, d4}, respectively. Assume A={d1:0.8, d2:0.7, d3:0.6, d4:0, d5:0, d6:0}

and B={d1:0, d2:0.6, d3:0.8, d4:0.9, d5:0, d6:0} = {d1:0.2, d2:0.3, d3:0.4, d4:1, d5:1, d6:1} =

{d1:0.8, d2:0.7, d3:0.8, d4:0.9, d5:0, d6:0} =

{d1:0, d2:0.6, d3:0.6, d4:0, d5:0, d6:0}

)(1)( uu AA ))(),(max()( uuu BABA

))(),(min()( uuu BABA

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Fuzzy Information Retrieval

basic idea– Expand the set of index terms in the query with related

terms (from the thesaurus) such that additional relevant documents can be retrieved

– A thesaurus can be constructed by defining a term-term correlation matrix c whose rows and columns are associated to the index terms in the document collection

keyword connection matrix

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Fuzzy Information Retrieval(Continued)

normalized correlation factor ci,l between two terms ki and kl (0~1)

In the fuzzy set associated to each index term ki, a document dj has a degree of membership µi,j

lili

lili nnn

nc

,

,,

)1(1 ,,

jdlk

liji c

where

ni is # of documents containing term ki

nl is # of documents containing term kl

ni,l is # of documents containing ki and kl

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Fuzzy Information Retrieval(Continued)

physical meaning– A document dj belongs to the fuzzy set associated to the

term ki if its own terms are related to ki, i.e., i,j=1.

– If there is at least one index term kl of dj which is strongly related to the index ki, then i,j1.

ki is a good fuzzy index

– When all index terms of dj are only loosely related to ki, i,j0.

ki is not a good fuzzy index

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Example

q = (ka (kb kc))= (ka kb kc) (ka kb kc) (ka kb kc)= cc1+cc2+cc3

Da

Db

Dc

cc3cc2

cc1

Da: the fuzzy set of documents associated to the index ka

djDa has a degree of membership a,j > a predefined threshold K

Da: the fuzzy set of documents associated to the index ka

(the negation of index term ka)

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Example

))1)(1(1())1(1()1(1

)1(1

,,,,,,,,,

3

1,

,321,

jcjbjajcjbjajcjbja

ijicc

jccccccjq

Query q=ka (kb kc)

disjunctive normal form qdnf=(1,1,1) (1,1,0) (1,0,0)

(1) the degree of membership in a disjunctive fuzzy set is computed using an algebraic sum (instead of max function) more smoothly(2) the degree of membership in a conjunctive fuzzy set is computed

using an algebraic product (instead of min function)

Recall )(1)( uu AA

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Fuzzy Set Model

– Q: “gold silver truck”D1: “Shipment of gold damaged in a fire”D2: “Delivery of silver arrived in a silver truck”D3: “Shipment of gold arrived in a truck”

– IDF (Select Keywords)

• a = in = of = 0 = log 3/3 arrived = gold = shipment = truck = 0.176 = log 3/2

damaged = delivery = fire = silver = 0.477 = log 3/1

– 8 Keywords (Dimensions) are selected

• arrived(1), damaged(2), delivery(3), fire(4), gold(5), silver(6), shipment(7), truck(8)

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Fuzzy Set Model

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Fuzzy Set Model

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Fuzzy Set Model

Sim(q,d): Alternative 1

Sim(q,d3) > Sim(q,d2) > Sim(q,d1) Sim(q,d): Alternative 2

Sim(q,d3) > Sim(q,d2) > Sim(q,d1)23

Extended Boolean Model

• Disadvantages of “Boolean Model” :• No term weight is used

• Counterexample: query q=Kx AND Ky.

Documents containing just one term, e,g, Kx is considered as

irrelevant as another document containing none of these terms.

• The size of the output might be too large or too small

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• The Extended Boolean model was introduced in 1983 by Salton, Fox, and Wu

• The idea is to make use of term weight as vector space model.

• Strategy: Combine Boolean query with vector space model.

• Why not just use Vector Space Model?• Advantages: It is easy for user to provide query.

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• Each document is represented by a vector (similar to vector space model.)

• Remember the formula.• Query is in terms of Boolean formula.• How to rank the documents?

ii

xjxjx

idf

idffw

max*,,

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Fig. Extended Boolean logic considering the space composed of two terms kx and ky only.

dj

dj +1dj +1

dj

kx and ky

kx or ky

( 0, 1) ( 0, 1)( 1, 1) ( 1, 1)

( 0, 0) ( 1, 0) ( 0, 0) ( 1, 0)

ky ky

kx kx27


• For query q=Kx or Ky, (0,0) is the point we try to avoid. Thus, we can use

to rank the documents• The bigger the better.

2),(

22 yxdqsim or

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• For query q=Kx and Ky, (1,1) is the most desirable point.

• We use

to rank the documents.• The bigger the better.

2

1(1),(

))1(22

yxdqsim and

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Extend the idea to m terms

• qor=k1 p k2 p … p Km

• qand=k1 p k2 p … p km

)...

( 21

/1

),(m

xxxp

m

pp p

jor dqsim

))1(...)1()1(

(1 21

/1

),(

m

xxx m

ppp

jand

p

dqsim

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Properties

• The p norm as defined above enjoys a couple of interesting properties as follows. First, when p=1 it can be verified that

• Second, when p= it can be verified that

• Sim(qor,dj)=max(xi)

• Sim(qand,dj)=min(xi)

m

xxdqsimdqsim

mjandjor

...),(),(

1

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Example

• For instance, consider the query q=(k1 k2) k3. The similarity sim(q,dj) between a document dj and this query is then computed as

• Any boolean can be expressed as a numeral formula.

)2

))(1((

321

/1/1

2

)1()1(),( x

pp p p

xxdqsim

pp

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