Chapter 7 Ontology Engineering

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Chapter 7Ontology Engineering

• Grigoris Antoniou
• Frank van Harmelen

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Lecture Outline

1. Introduction
2. Constructing Ontologies Manually
3. Reusing Existing Ontologies
4. Semiautomatic Ontology Acquisition
5. Ontology Mapping
6. On-To-Knowledge SW Architecture

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Methodological Questions

• How can tools and techniques best be applied?
• Which languages and tools should be used in which
  circumstances, and in which order?
• What about issues of quality control and resource
  management?
• Many of these questions for the Semantic Web have
  been studied in other contexts
• E.g. software engineering, object-oriented
  design, and knowledge engineering

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Lecture Outline

1. Introduction
2. Constructing Ontologies Manually
3. Reusing Existing Ontologies
4. Semiautomatic Ontology Acquisition
5. Ontology Mapping
6. On-To-Knowledge SW Architecture

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Main Stages in Ontology Development

• Determine scope
• Consider reuse
• Enumerate terms
• Define taxonomy
• Define properties
• Define facets
• Define instances
• Check for anomalies
• Not a linear process!

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Determine Scope

• There is no correct ontology of a specific domain
  
• An ontology is an abstraction of a particular
  domain, and there are always viable alternatives
• What is included in this abstraction should be
  determined by
• the use to which the ontology will be put
• by future extensions that are already anticipated

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Determine Scope (2)

• Basic questions to be answered at this stage are
  
• What is the domain that the ontology will cover?
• For what we are going to use the ontology?
• For what types of questions should the ontology
  provide answers?
• Who will use and maintain the ontology?

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Consider Reuse

• With the spreading deployment of the Semantic
  Web, ontologies will become more widely available
  
• We rarely have to start from scratch when
  defining an ontology
• There is almost always an ontology available from
  a third party that provides at least a useful
  starting point for our own ontology

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Enumerate Terms

• Write down in an unstructured list all the
  relevant terms that are expected to appear in the
  ontology
• Nouns form the basis for class names
• Verbs (or verb phrases) form the basis for
  property names
• Traditional knowledge engineering tools (e.g.
  laddering and grid analysis) can be used to
  obtain
• the set of terms
• an initial structure for these terms

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Define Taxonomy

• Relevant terms must be organized in a taxonomic
  hierarchy
• Opinions differ on whether it is more
  efficient/reliable to do this in a top-down or a
  bottom-up fashion
• Ensure that hierarchy is indeed a taxonomy
• If A is a subclass of B, then every instance of A
  must also be an instance of B (compatible with
  semantics of rdfssubClassOf

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Define Properties

• Often interleaved with the previous step
• The semantics of subClassOf demands that whenever
  A is a subclass of B, every property statement
  that holds for instances of B must also apply to
  instances of A
• It makes sense to attach properties to the
  highest class in the hierarchy to which they
  apply

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Define Properties (2)

• While attaching properties to classes, it makes
  sense to immediately provide statements about the
  domain and range of these properties
• There is a methodological tension here between
  generality and specificity
• Flexibility (inheritance to subclasses)
• Detection of inconsistencies and misconceptions

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Define Facets From RDFS to OWL

• Cardinality restrictions
• Required values
• owlhasValue
• owlallValuesFrom
• owlsomeValuesFrom
• Relational characteristics
• symmetry, transitivity, inverse properties,
  functional values

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Define Instances

• Filling the ontologies with such instances is a
  separate step
• Number of instances gtgt number of classes
• Thus populating an ontology with instances is not
  done manually
• Retrieved from legacy data sources (DBs)
• Extracted automatically from a text corpus

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Check for Anomalies

• An important advantage of the use of OWL over RDF
  Schema is the possibility to detect
  inconsistencies
• In ontology or ontologyinstances
• Examples of common inconsistencies
• incompatible domain and range definitions for
  transitive, symmetric, or inverse properties
• cardinality properties
• requirements on property values can conflict with
  domain and range restrictions

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Lecture Outline

1. Introduction
2. Constructing Ontologies Manually
3. Reusing Existing Ontologies
4. Semiautomatic Ontology Acquisition
5. Ontology Mapping
6. On-To-Knowledge SW Architecture

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Existing Domain-Specific Ontologies

• Medical domain Cancer ontology from the National
  Cancer Institute in the United States
• Cultural domain
• Art and Architecture Thesaurus (AAT) with
  125,000 terms in the cultural domain
• Union List of Artist Names (ULAN), with 220,000
  entries on artists
• Iconclass vocabulary of 28,000 terms for
  describing cultural images
• Geographical domain Getty Thesaurus of
  Geographic Names (TGN), containing over 1 million
  entries

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Integrated Vocabularies

• Merge independently developed vocabularies into a
  single large resource
• E.g. Unified Medical Language System
  integrating100 biomedical vocabularies
• The UMLS metathesaurus contains 750,000 concepts,
  with over 10 million links between them
• The semantics of a resource that integrates many
  independently developed vocabularies is rather
  low
• But very useful in many applications as starting
  point

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Upper-Level Ontologies

• Some attempts have been made to define very
  generally applicable ontologies
• Mot domain-specific
• Cyc, with 60,000 assertions on 6,000 concepts
• Standard Upperlevel Ontology (SUO)

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Topic Hierarchies

• Some ontologies do not deserve this name
• simply sets of terms, loosely organized in a
  hierarchy
• This hierarchy is typically not a strict taxonomy
  but rather mixes different specialization
  relations (e.g. is-a, part-of, contained-in)
• Such resources often very useful as starting
  point
• Example Open Directory hierarchy, containing
  more then 400,000 hierarchically organized
  categories and available in RDF format

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Linguistic Resources

• Some resources were originally built not as
  abstractions of a particular domain, but rather
  as linguistic resources
• These have been shown to be useful as starting
  places for ontology development
• E.g. WordNet, with over 90,000 word senses

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Ontology Libraries

• Attempts are currently underway to construct
  online libraries of online ontologies
• Rarely existing ontologies can be reused without
  changes
• Existing concepts and properties must be refined
  using rdfssubClassOf and rdfssubPropertyOf
• Alternative names must be introduced which are
  better suited to the particular domain using
  owlequivalentClass and owlequivalentProperty
• We can exploit the fact that RDF and OWL allow
  private refinements of classes defined in other
  ontologies

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Lecture Outline

1. Introduction
2. Constructing Ontologies Manually
3. Reusing Existing Ontologies
4. Semiautomatic Ontology Acquisition
5. Ontology Mapping
6. On-To-Knowledge SW Architecture

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The Knowledge Acquisition Bottleneck

• Manual ontology acquisition remains a
  time-consuming, expensive, highly skilled, and
  sometimes cumbersome task
• Machine Learning techniques may be used to
  alleviate
• knowledge acquisition or extraction
• knowledge revision or maintenance

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Tasks Supported by Machine Learning

• Extraction of ontologies from existing data on
  the Web
• Extraction of relational data and metadata from
  existing data on the Web
• Merging and mapping ontologies by analyzing
  extensions of concepts
• Maintaining ontologies by analyzing instance data
• Improving SW applications by observing users

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Useful Machine Learning Techniques for Ontology
Engineering

• Clustering
• Incremental ontology updates
• Support for the knowledge engineer
• Improving large natural language ontologies
• Pure (domain) ontology learning

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Machine Learning Techniques for Natural Language
Ontologies

• Natural language ontologies (NLOs) contain
  lexical relations between language concepts
• They are large in size and do not require
  frequent updates
• The state of the art in NLO learning looks quite
  optimistic
• A stable general-purpose NLO exist
• Techniques for automatically or
  semi-automatically constructing and enriching
  domain-specific NLOs exist

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Machine Learning Techniques for Domain Ontologies

• They provide detailed descriptions
• Usually they are constructed manually
• The acquisition of the domain ontologies is still
  guided by a human knowledge engineer
• Automated learning techniques play a minor role
  in knowledge acquisition
• They have to find statistically valid
  dependencies in the domain texts and suggest them
  to the knowledge engineer

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Machine Learning Techniques for Ontology Instances

• Ontology instances can be generated automatically
  and frequently updated while the ontology remains
  unchanged
• Fits nicely into a machine learning framework
• Successful ML applications
• Are strictly dependent on the domain ontology, or
  
• Populate the markup without relating to any
  domain theory
• General-purpose techniques not yet available

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Different Uses of Ontology Learning

• Ontology acquisition tasks in knowledge
  engineering
• Ontology creation from scratch by the knowledge
  engineer
• Ontology schema extraction from Web documents
• Extraction of ontology instances from Web
  documents
• Ontology maintenance tasks
• Ontology integration and navigation
• Updating some parts of an ontology
• Ontology enrichment or tuning

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Ontology Acquisition Tasks

• Ontology creation from scratch by the knowledge
  engineer
• ML assists the knowledge engineer by suggesting
  the most important relations in the field or
  checking and verifying the constructed knowledge
  bases
• Ontology schema extraction from Web documents
• ML takes the data and meta-knowledge (like a
  meta-ontology) as input and generate the
  ready-to-use ontology as output with the possible
  help of the knowledge engineer

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Ontology Acquisition Tasks(2)

• Extraction of ontology instances from Web
  documents
• This task extracts the instances of the ontology
  presented in the Web documents and populates
  given ontology schemas
• This task is similar to information extraction
  and page annotation, and can apply the techniques
  developed in these areas

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Ontology Maintenance Tasks

• Ontology integration and navigation
• Deals with reconstructing and navigating in large
  and possibly machine-learned knowledge bases
• Updating some parts of an ontology that are
  designed to be updated
• Ontology enrichment or tuning
• This does not change major concepts and
  structures but makes an ontology more precise

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Potentially Applicable Machine Learning Algorithms

• Propositional rule learning algorithms
• Bayesian learning
• generates probabilistic attribute-value rules
• First-order logic rules learning
• Clustering algorithms
• They group the instances together based on the
  similarity or distance measures between a pair of
  instances defined in terms of their attribute
  values

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Lecture Outline

1. Introduction
2. Constructing Ontologies Manually
3. Reusing Existing Ontologies
4. Semiautomatic Ontology Acquisition
5. Ontology Mapping
6. On-To-Knowledge SW Architecture

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Ontology Mapping

• A single ontology will rarely fulfill the needs
  of a particular application multiple ontologies
  will have to be combined
• This raises the problem of ontology integration
  (also called ontology alignment or ontology
  mapping)
• Current approaches deploy a whole host of
  different methods we distinguish linguistic,
  statistical, structural and logical methods

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Linguistic methods

• The most basic methods try to exploit the
  linguistic labels attached to the concepts in
  source and target ontology in order to discover
  potential matches
• This can be as simple as basic stemming
  techniques or calculating Hamming distances, or
  it can use specialized domain knowledge (e.g. the
  difference between Diabetes Melitus type I and
  Diabetes Melitus type II is not a negligible
  difference to be removed by a small Hamming
  distance)

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Statistical Methods

• Some methods use instance data, to determine
  correspondences between concepts
• A significant statistical correlation between the
  instances of a source concept and a target
  concept, gives us reason to believe that these
  concepts are strongly related
• These approaches rely on the availability of a
  sufficiently large corpus of instances that are
  classified in both the source and the target
  ontologies

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Structural Methods

• Since ontologies have internal structure, it
  makes sense to exploit the graph structure of the
  source and the target ontologies and try to
  determine similarities, often in coordination
  with other methods
• If a source target and a target concept have
  similar linguistic labels, then the dissimilarity
  of their graph neighborhoods could be used to
  detect homonym problems where purely linguistic
  methods would falsely declare a potential mapping

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Logical Methods

• The most specific to mapping ontologies
• A serious limitation of this approach is that
  many practical ontologies are semantically rather
  lightweight and thus dont carry much logical
  formalism with them

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Ontology-Mapping Techniques Conclusion

• Although there is much potential, and indeed
  need, for these techniques to be deployed for
  Semantic Web engineering, this is far from a
  well-understood area
• No off-the-shelf techniques are currently
  available, and it is not clear that this is
  likely to change in the near future

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Lecture Outline

1. Introduction
2. Constructing Ontologies Manually
3. Reusing Existing Ontologies
4. Semiautomatic Ontology Acquisition
5. Ontology Mapping
6. On-To-Knowledge SW Architecture

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On-To-Knowledge Architecture

• Building the Semantic Web involves using
• the new languages described in this course
• a rather different style of engineering
• a rather different approach to application
  integration
• We describe how a number of Semantic Web-related
  tools can be integrated in a single lightweight
  architecture using Semantic Web standards to
  achieve interoperability between tools

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Knowledge Acquisition

• Initially, tools must exist that use surface
  analysis techniques to obtain content from
  documents
• Unstructured natural language documents
  statistical techniques and shallow natural
  language technology
• Structured and semi-structured documents
  wrappers induction, pattern recognition

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Knowledge Storage

• The output of the analysis tools is sets of
  concepts, organized in a shallow concept
  hierarchy with at best very few cross-taxonomical
  relationships
• RDF/RDF Schema are sufficiently expressive to
  represent the extracted info
• Store the knowledge produced by the extraction
  tools
• Retrieve this knowledge, preferably using a
  structured query language (e.g. RQL)

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Knowledge Maintenance and Use

• A practical Semantic Web repository must provide
  functionality for managing and maintaining the
  ontology
• change management
• access and ownership rights
• transaction management
• There must be support for both
• Lightweight ontologies that are automatically
  generated from unstructured and semi-structured
  data
• Human engineering of much more knowledge-intensive
  ontologies

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Knowledge Maintenance and Use (2)

• Sophisticated editing environments must be able
  to
• Retrieve ontologies from the repository
• Allow a knowledge engineer to manipulate it
• Place it back in the repository
• The ontologies and data in the repository are to
  be used by applications that serve an end-user
• We have already described a number of such
  applications

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Technical Interoperability

• Syntactic interoperability was achieved because
  all components communicated in RDF
• Semantic interoperability was achieved because
  all semantics was expressed using RDF Schema
• Physical interoperability was achieved because
• All communications between components were
  established using simple HTTP connections

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On-To-Knowledge System Architecture