Information Extractionfrom Web Documents

CS 652 Information Extraction and Integration

Li XuYihong Ding

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IR and IEIR (Information Retrieval) Retrieves relevant documents from collections Information theory, probabilistic theory, and

statistics

IE (Information Extraction) Extracts relevant information from documents Machine learning, computational linguistics,

and natural language processing

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History of IE

Large amount of both online and offline textual data.Message Understanding Conference (MUC) Quantitative evaluation of IE systems Tasks

Latin American terrorism Joint ventures Microelectronics Company management changes

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Evaluation MetricsPrecision

Recall

F-measure

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Web Documents

Unstructured (Free) Text Regular sentences and paragraphs Linguistic techniques, e.g., NLP

Structured Text Itemized information Uniform syntactic clues, e.g., table

understanding

Semistructured Text Ungrammatical, telegraphic (e.g., missing

attributes, multi-value attributes, …) Specialized programs, e.g., wrappers

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Approaches to IEKnowledge Engineering Grammars are constructed by hand Domain patterns are discovered by human

experts through introspection and inspection of a corpus

Much laborious tuning and “hill climbing”

Machine Learning Use statistical methods when possible Learn rules from annotated corpora Learn rules from interaction with user

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Knowledge EngineeringAdvantages With skills and experience, good performing

systems are not conceptually hard to develop.

The best performing systems have been hand crafted.

Disadvantages Very laborious development process Some changes to specifications can be hard

to accommodate Required expertise may not be available

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Machine Learning Advantages Domain portability is relatively straightforward System expertise is not required for customization “Data driven” rule acquisition ensures full

coverage of examples

Disadvantages Training data may not exist, and may be very

expensive to acquire Large volume of training data may be required Changes to specifications may require

reannotation of large quantities of training data

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WrapperA specialized program that identifies data of interest and maps them to some suitable format (e.g. XML or relational tables)

Challenge: recognizing the data of interest among many other uninterested pieces of text

Tasks Source understanding Data processing

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Free Text

AutoSlogLiepPalkaHastenCrystal WebFoot

WHISK

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AutoSlog [1993]

The Parliament building was bombed by Carlos.

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LIEP [1995]

The Parliament building was bombed by Carlos.

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PALKA [1995]

The Parliament building was bombed by Carlos.

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HASTEN [1995]

The Parliament building was bombed by Carlos.

Egraphs(SemanticLabel, StructuralElement)

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CRYSTAL [1995]The Parliament building was bombed by Carlos.

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CRYSTAL + Webfoot [1997]

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WHISK [1999]The Parliament building was bombed by Carlos.

WHISK Rule:*(PhyObj)*@passive *F ‘bombed’ * {PP

‘by’ *F (Person)}

Context-based patterns

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Web DocumentsSemistructured and Unstructured RAPIER (E. Califf, 1997) SRV (D. Freitag, 1998) WHISK (S. Soderland, 1998)

Semistructured and Structured WIEN (N. Kushmerick, 1997) SoftMealy (C-H. Hsu, 1998) STALKER (I. Muslea, S. Minton, C. Knoblock,

1998)

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Inductive Learning

TaskInductive InferenceLearning Systems Zero-order First-order, e.g., Inductive Logic

Programming (ILP)

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RAPIER [1997]Inductive Logic ProgrammingExtraction Rules Syntactic information Semantic information

Advantage Efficient learning (bottom-up)

Drawback Single-slot extraction

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RAPIER Rule

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SRV [1998]Relational Algorithm (top-down)Features Simple features (e.g., length, character

type, …) Relational features (e.g., next-token, …)

Advantages Expressive rule representation

Drawbacks Single-slot rule generation Large-volume of training data

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SRV Rule

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WHISK [1998]Covering Algorithm (top-down)Advantages Learn multi-slot extraction rules Handle various order of items-to-be-extracted Handle document types from free text to

structured text

Drawbacks Must see all the permutations of items Less expressive feature set Need large volume of training data

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WHISK Rule

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WIEN [1997]Assumes Items are always in fixed, known order

Introduces several types of wrappersAdvantages Fast to learn and extract

Drawbacks Can not handle permutations and missing

items Must label entire pages Does not use semantic classes

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WIEN Rule

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SoftMealy [1998]Learns a transducerAdvantages Learns order of items Allows item permutations and missing items Allows both the use of semantic classes and

disjunctions

Drawbacks Must see all possible permutations Can not use delimiters that do not

immediately precede and follow the relevant items

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SoftMealy Rule

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STALKER [1998,1999,2001]

Hierarchical Information ExtractionEmbedded Catalog Tree (ECT) FormalismAdvantages Extracts nested data Allows item permutations and missing items Need not see all of the permutations One hard-to-extract item does not affect others

Drawbacks Does not exploit item order

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STALKER Rule

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Web IE Tools (main technique used)

Wrapper languages (TSIMMIS, Web-OQL) HTML-aware (X4F, XWRAP, RoadRunner, Lixto) NLP-based (RAPIER, SRV, WHISK) Inductive learning (WIEN, SoftMealy, Stalker) Modeling-based (NoDoSE, DEByE) Ontology-based (BYU ontology)

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Degree of Automation

Trade-off: page lay-out dependent

RoadRunner Assume target pages were automatically

generated from some data sources The only fully automatic wrapper generator

BYU ontology Manually created with graphical editing tool Extraction process fully automatic

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Support of Complex Objects

Complex objects: nested objects, graphs, trees, complex tables, …

Earlier tools do not support extracting from complex objects, like RAPIER, SRV, WHISK, and WIEN.BYU ontology Support

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Page Contents

Semistructured data (table type, richly tagged)Semistructured text (text type, rarely tagged)

NLP-based tools: text type onlyOther tools (except ontology-based): table type onlyBYU ontology: both types

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Ease of Use

HTML-aware tools, easiest to use

Wrapper languages, hardest to use

Other tools, in the middle

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Output

XML is the best output format for data sharing on the Web.

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Support for Non-HTML Sources

NLP-based and ontology-based, automatically supportOther tools, may support but need additional helper like syntactical and semantic analyzer

BYU ontology support

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Resilience and Adaptiveness

Resilience: continuing to work properly in the occurrence of changes in the target pagesAdaptiveness: working properly with pages from some other sources but in the same application domain

Only BYU ontology has both the features.

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Summary of Qualitative Analysis

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Graphical Perspective of Qualitative Analysis

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Name Struc_ture

Semi

Free Single-slot

Multi-slot

Missing items

Permuta_tions

Nested_data

Resilient

WIEN X X X

SoftMealy

X X X X X X*

STALKER

X X X * X X X

RAPIER X X ? X X X ?SRV X X ? X X X ?

WHISK X X X X X X X* ?

AutoSlog

X X X X

ROAD_RUNNER

X X X X X

BYU Onto

X X ? X X X X X X

X means the information extraction system has the capability; X* means the information extraction system has the ability as long as the training corpus can accommodate the required training data; ? Shows that the systems can has the ability in somewhat degree; * means that the extraction pattern itself doesn’t show the ability, but the overall system has the capability.

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Problem of IE (unstructured documents)

Meaning

Knowledge

Information

Data

Source Target

Information Extraction

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Problem of IE (structured documents)

Meaning

Knowledge

Information

Data

Source Target

Information Extraction

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Problem of IE (semistructured documents)

Meaning

Knowledge

Information

Data

Source Target

Information Extraction

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Meaning

Knowledge

Information

Data

Solution of IE (the Semantic Web)

Source Target

Information Extraction