Recuperação de Informação B

Cap. 06: Text and Multimedia Languages and

Properties (Introduction, Metadata

and Text)

6.1, 6.2, 6.3

November 01, 1999

Introduction

Text main form of communicating knowledge.

Document loosely defined, denote a single unit of information. can be any physical unit

a file an email a Web Page

Introduction

Document Syntax and structure Semantics Information about itself

Introduction

Document Syntax Implicit, or expressed in a language (e.g, TeX) Powerful languages: easier to parse, difficult to convert

to other formats. Open languages are better (interchange) Semantics of texts in natural language are not easy for

a computer to understand Trend: languages which provides information on

structure, format and semantics being readable by human and computers

Introduction

New applications are pushing for format such that information can be represented independetly of style.

Style: defined by the author, but the reader may decide part of it

Style can include treatment of other media

Metadata “Data about the data”

e.g: in a DBMS, schema specifies name of the relations, attributes, domains, etc.

Descriptive Metadata Author, source, length Dublin Core Metadata Element Set

Semantic Metadata Characterizes the subject matter within the document

contents MEDLINE

Metadata MARC

100 0020 1 $aHagler, Ronald.

245 0074 14$aThe bibliographic...

250 0012 $a3rd. Ed.

260 0052 $aChicago :$bALA, $c1997

Metadata Metadata information on Web documents

cataloging, content rating, property rights, digital signatures

New standard: Resource Description Framework description of Web resources to facilitate

automated processing of information nodes and attched atribute/values pairs

Metadescription of non-textual objects keyword can be used to search the objects

Metadata RDF Example

<RDF:RDF>

<RDF:Description RDF:HREF = “page.html”> <DC:Creator> John Smith </DC:Creator> <DC:Title> John’s Home Page </DC:Title> </RDF:Description>

</RDF:RDF>

Metadata RDF Schema Exemple

Text Text coding in bits

EBCDIC, ASCII Initially, 7 bits. Later, 8 bits

Unicode 16 bits, to accommodate oriental languages

Text Formats

No single format exists IR system should retrieve information from different

formats Past: IR systems convert the documents Today: IR systems use filters

Text Formats

Formats for document interchange (RTF) Formats for displaying (PDF, PostScript) Formats for encode email (MIME) Compressed files

uuencode/uudecode, binhex

Text Information Theory

Amount of information is related to the distribution of symbols in the document.

Entropy:

Definition of entropy depends on the probabilities of each symbol.

Text models are used to obtain those probabilites

ii

i ppE 21

log

Text Example - Entropy

001001011011

12

1log2

1

2

1log2

122

E

Text Example - Entropy

111111111111

01log10log0 22 E

Text Modeling Natural Language

Symbols: separate words or belong to words Symbols are not uniformly distributed

binomial model Dependency of previous symbols

k-order markovian model We can take words as symbols

Text Modeling Natural Language

Words distribution inside documents Zipf´s Law: i-th most frequent word appears 1/i times of

the most frequent word

Real data fits better with between 1.5 and 2.0

V

jV

V

jH

Hin

1

1)(

))(/(

Text Modeling Natural Language

Example - word distibution (Zipf’s Law) V=1000, = 2 most frequent word: n=300 2nd most frequent: n=76 3rd most frequent: n=33 4th most frequent: n=19

Text Modeling Natural Language

Skewed distribution - stopwords Distribution of words in the documents

binomial distribution

Poisson distribution

kk ppk

kkF

)1(

1)(

Text Modeling Natural Language

Number of distinct words Heaps’ Law: Set of different words is fixed by a constant, but the

limit is too high

KnV

Text Modeling Natural Language

Heaps’ Law example k between 10 and 100, is less than 1 example: n=400000, = 0.5

• K=25, V=15811• K=35, V=22135

Text Modeling Natural Language

Length of the words defines total space needed for vocabulary

Heaps’ Law: length increases logarithmically with text size.

In practice, a finit-state model is used space has p=0.2 space cannot apear twice subsequently there are 26 letters

Text Similarity Models

Distance Function Should be symmetric and satisfy triangle inequality

Hamming Distance number of positions that have different characters

reverse

receive

Text Similarity Models

Edit (Levenshtein) Distance minimum number of operations needed to make strings

equal

survey

surgery

superior for modeling syntatic errors extensions: weights, transpositions, etc

Text Similarity Models

Longest Common Subsequence (LCS) survey - surgery

LCS: surey Documents: lines as symbols (diff in Unix)

time consuming similar lines

Fingerprints Visual tools

Conclusions Text is the main form of communicating knowledge. Documents have syntax, structure and semantics Metadata: information about data Formats of text Modeling Natural Language

Entropy Distribution of symbols

Similarity