ontology engineering & maintenance semantic web - spring 2008 computer engineering department...
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Ontology Engineering & Maintenance
Semantic Web - Spring 2008
Computer Engineering Department
Sharif University of Technology
Introduction Why do we use ontology?
To describe the semantics of the data (which we name as Meta-Data)
Why do we describe the semantics? In order to provide a uniform way to make different
parties to understand each other
Which data? Any data (on the web, or in the existing legacy
databases)
Introduction Formal definition on Ontology:
Ontologies are knowledge bodies that provide a formal representation of a shared conceptualization of a particular domain.
Ontologies are widely used in the Semantic Web. Recently ontologies have become
increasingly common on WWW where they provide semantics of annotations in web pages
What Is “Ontology Engineering”?Ontology Engineering: Defining terms in the
domain and relations among them Defining concepts in the domain (classes) Arranging the concepts in a hierarchy
(subclass-superclass hierarchy) Defining which attributes and properties (slots)
classes can have and constraints on their values
Defining individuals and filling in slot values
Ontology-Development Processhere:
determinescope
considerreuse
enumerateterms
defineclasses
defineproperties
defineconstraints
createinstances
In reality - an iterative process:
determinescope
considerreuse
enumerateterms
defineclasses
considerreuse
enumerateterms
defineclasses
defineproperties
createinstances
defineclasses
defineproperties
defineconstraints
createinstances
defineclasses
considerreuse
defineproperties
defineconstraints
createinstances
Determine Domain and Scope
What is the domain that the ontology will cover?
For what we are going to use the ontology? For what types of questions the information in
the ontology should provide answers?
determinescope
considerreuse
enumerateterms
defineclasses
defineproperties
defineconstraints
createinstances
Consider Reuse
Why reuse other ontologies? to save the effort to interact with the tools that use other
ontologies to use ontologies that have been validated
through use in applications
determinescope
considerreuse
enumerateterms
defineclasses
defineproperties
defineconstraints
createinstances
What to Reuse? Ontology libraries
DAML ontology library (www.daml.org/ontologies) Ontolingua ontology library
(www.ksl.stanford.edu/software/ontolingua/) Protégé ontology library
(protege.stanford.edu/plugins.html)
Upper ontologies IEEE Standard Upper Ontology (suo.ieee.org) Cyc (www.cyc.com)
What to Reuse? (II) General ontologies
DMOZ (www.dmoz.org)
WordNet (www.cogsci.princeton.edu/~wn/) Domain-specific ontologies
UMLS Semantic Net GO (Gene Ontology) (www.geneontology.org)
Enumerate Important Terms
What are the terms we need to talk about? What are the properties of these terms? What do we want to say about the terms?
considerreuse
determinescope
enumerateterms
defineclasses
defineproperties
defineconstraints
createinstances
Define Classes and the Class Hierarchy
A class is a concept in the domain a class of wines a class of wineries a class of red wines
A class is a collection of elements with similar properties
Instances of classes a glass of California wine you’ll have for lunch
considerreuse
determinescope
defineclasses
defineproperties
defineconstraints
createinstances
enumerateterms
Classes usually constitute a taxonomic hierarchy (a subclass-superclass hierarchy)
A class hierarchy is usually an IS-A hierarchy:
an instance of a subclass is an instance of a superclass
If you think of a class as a set of elements, a subclass is a subset
e.g., Apple is a subclass of FruitEvery apple is a fruit
Class Inheritance
Modes of Development top-down – define the most general
concepts first and then specialize them bottom-up – define the most specific
concepts and then organize them in more general classes
combination – define the more salient concepts first and then generalize and specialize them
Documentation Classes (and Properties) usually have
documentation Describing the class in natural language Listing domain assumptions relevant to the
class definition Listing synonyms
Documenting classes and slots is as important as documenting computer code!
Define Properties (Slots) of Classes
Properties in a class definition describe attributes of instances of the class and relations to other instancesEach wine will have color, sugar content,
producer, etc.
considerreuse
determinescope
defineconstraints
createinstances
enumerateterms
defineclasses
defineproperties
Properties (Slots) Types of properties
“intrinsic” properties: flavor and color of wine “extrinsic” properties: name and price of wine parts: ingredients in a dish relations to other objects: producer of wine (winery)
Simple and complex properties simple properties (attributes): contain primitive values
(strings, numbers) complex properties: contain (or point to) other objects
(e.g., a winery instance)
Property Constraints (facets)
Property constraints (facets) describe or limit the set of possible values for a property
The name of a wine is a stringThe wine producer is an instance of WineryA winery has exactly one location
considerreuse
determinescope
createinstances
enumerateterms
defineclasses
defineconstraints
defineproperties
An Example: Domain and Range
When defining a domain or range for a slot, find the most general class or classes
Consider the flavor slot Domain: Red wine, White wine, Rosé wine Domain: Wine
Consider the produces slot for a Winery: Range: Red wine, White wine, Rosé wine Range: Wine
slotclass allowed values
DOMAIN RANGE
Create Instances
Create an instance of a class The class becomes a direct type of the instance Any superclass of the direct type is a type of the
instance Assign slot values for the instance frame
Slot values should conform to the facet constraints Knowledge-acquisition tools often check that
considerreuse
determinescope
createinstances
enumerateterms
defineclasses
defineproperties
defineconstraints
Defining Classes and a Class Hierarchy The things to remember:
There is no single correct class hierarchy But there are some guidelines
The question to ask:“Is each instance of the subclass an instance of
its superclass?”
Transitivity of the Class Hierarchy
The is-a relationship is transitive:B is a subclass of AC is a subclass of BC is a subclass of A
A direct superclass of a class is its “closest” superclass
Multiple Inheritance A class can have more than
one superclass A subclass inherits slots and
facet restrictions from all the parents
Different systems resolve conflicts differently
Disjoint Classes
Classes are disjoint if they cannot have common instances Disjoint classes cannot have any common subclasses either
Red wine, White wine,Rosé wine are disjoint
Dessert wine and Redwine are not disjoint
Wine
Redwine
Roséwine
Whitewine
Dessertwine
Avoiding Class Cycles Danger of multiple
inheritance: cycles in the class hierarchy
Classes A, B, and C have equivalent sets of instances By many definitions, A, B, and C
are thus equivalent
The Perfect Family Size If a class has only one child,
there may be a modeling problem
If the only Red Burgundy we have is Côtes d’Or, why introduce the sub-hierarchy?
Compare to bullets in a bulleted list
The Perfect Family Size (II) If a class has more
than a dozen children, additional subcategories may be necessary
However, if no natural classification exists, the long list may be more natural
Single and Plural Class Names A “wine” is not a kind-of
“wines” A wine is an instance of the
class Wines Class names should be either
all singular all plural
Class
Instance
instance-of
Classes and Their Names Classes represent concepts in the domain, not
their names The class name can change, but it will still refer
to the same concept Synonym names for the same concept are not
different classes Many systems allow listing synonyms as part of the class
definition
Content: Top-Level Ontologies What does “top-level” mean?
Objects: tangible, intangible Processes, events, actors, roles Agents, organizations Spaces, boundaries, location Time
IEEE Standard Upper Ontology effort Goal: Design a single upper-level ontology Process: Merge upper-level of existing ontologies
Ontology Evaluation Key factor which makes a particular discipline or
approach scientific is the ability to evaluate and compare the ideas within the area.
In most practical cases ontologies are a non-uniquely expressible.
One can build many different ontologies which conceptualizing the same body of knowledge.
We should be able to say which of these ontologies serves better some predefined criterion.
Categories of Ontology Evaluation Those based on comparing the ontology to a
"golden standard“ (a ontology). Those based on using the ontology in an
application and evaluating the results of it. Those involving comparisons with a source of
data (e.g. a collection of documents) about the domain that is to be covered by the ontology.
Those where evaluation is done by humans who try to assess how well the ontology meets a set of predefined criteria, standards, requirements, etc.
Different Levels of Evaluation Lexical, vocabulary, or Data Layer Hierarchy or Taxonomy Other Semantic relations Context or application level Syntactic Level Structure, Architecture, Design Multiple-criteria approaches
A: Lexical, Vocabulary, or Data Layer The focus is on which concepts, instances, facts, etc. have
been include in the ontology, and the vocabulary used to represent or identify these concepts.
Evaluation on this level tends to involve comparisons with various sources of data concerning the problem, as well as techniques such as string similarity measures (e.g. edit distance).
MAEDCHE AND STAAB (2002). Concepts are compared to a “Golden Standard” set of strings that are considered a good representation of the concepts.
Golden standard Another ontology Taken statistically from a corpus of documents Prepared by domain experts.
B: Hierarchy or Taxonomy An ontology typically includes a hierarchical “is-a
or subsumption” relation between concepts. BREWSTER et al. (2004) used a data-driven
approach to evaluate the degree of structural fit between an ontology and a corpus of documents. Cluster the documents and make topic representing
documents Each concept c of the ontology is represented by a set of
terms including its name in the ontology and the hypernyms of this name, taken from Wordnet.
Measure how well a concept fits a topic results from the clustering step.
Indicate that the structure of the ontology is reasonably well aligned with the hidden structure of topics in the domain-specific corpus of documents.
C: Context Level An ontology may be part of a larger collection of ontologies,
and may reference or be referenced by various definitions in these other ontologies. In this case it may be important to take this context into account when evaluating it.
Swoogle search engine uses cross-references between semantic-web documents to define a graph and compute a score for each ontology in a manner analogous to PageRank used by the Google web search engine. The resulting “ontology rank” is used by Swoogle to rank its query results.
An important difference in comparison to PageRank is that not all “links” or references between ontologies are treated the same. If one ontology defines a subclass of a class from another ontology, this reference might be considered more important than if one ontology only uses a class from another as the domain or range of some relation.
D: Application Level It may be more practical to evaluate an ontology
within the context of particular application, and to see how the results of the application are affected by the use of ontology in question.
The outputs of the application, or its performance on the given task, might be better or worse depending partly on the ontology used in it.
One might argue that a good ontology is one which helps the application in question produce good results on the given task.
E: Syntactic Level For manually constructed Ontologies. The ontology is usually described in a
particular formal language and must match the syntactic requirements of that language (use of the correct keywords, etc.).
This is probably the one that lends itself the most easily to automated processing.
F: Structure, Architecture, Design This is primarily of interest in manually
constructed ontologies. Assuming that some kind of design
principles or criteria have been agreed upon prior to constructing the ontology, evaluation on this level means checking to what extent the resulting ontology matches those criteria.
Must usually be done largely or even entirely manually by people such as ontological engineers and domain experts.
G: Multiple-Criteria Approaches Selecting a good ontology from a given set of
ontologies. Techniques familiar from the area of decision
support systems can be used to help us evaluate the ontologies and choose one of them.
Are based on defining several decision criteria or attributes; for each criterion, the ontology is evaluated and given a
numerical score. A weight is assigned to each criterion. An overall score for the ontology is then computed as a
weighted sum of its per-criterion scores.
Example Select an Ontology - Type G: Ontology Auditor Metrics Suite
Metric Attributes DescriptionSyntactic Quality
Lawfulness Correctness of syntax used
Richness Breadth of syntax used
Semantic Quality
Interpretability Meaningfulness of terms
Consistency Consistency of meaning of terms
Clarity Average number of word senses
Pragmatic Quality
Comprehensibility Amount of information
Accuracy Accuracy of information
Relevance Relevance of information for a task
Social QualityAuthority Extent to which other ontologies rely on it
History Number of times ontology has been used
Example Cont.: Overall Quality Metric Overall quality (Q) is a weighted function of its
constituents: Q = c1 × S + c2 × E + c3 × P + c4 × O where S = syntactic quality E = semantic quality P = pragmatic quality O = social quality, and c1+c2+c3+c4 = 1
The weights sum to unity, and currently, are set by the user, the application, or else assumed equal
Example Cont.: Syntactic Quality (S) Measures the quality of the ontology
according to the way it is written. Lawfulness
refers to the degree to which an ontology language’s rules have been complied.
Richness refers to the proportion of features in the ontology
language that have been used in an ontology
Syntactic Quality (S) S = b1SL + b2SR
Lawfulness (SL)Let X be total syntactical rules. Let Xb be total breached rules. Let NS
be the number of statements in the ontology. Then SL = Xb / NS.
Richness (SR)Let Y be the total syntactical features available in ontology language. Let Z be the total syntactical features used in this ontology. Then SR = Z/Y.
Example Cont.: Semantic Quality (E) Evaluates the meaning of terms in the
ontology library. Interpretability
refers to the meaning of terms in the ontology Consistency
whether terms have consistent meaning Clarity
whether the context of terms is clear
Semantic Quality (E) E = b1EI + b2EC + b3EA
Interpretability (EI)Let C be the total number of terms used to define classes and properties in ontology. Let W be the number of terms that have a sense listed in WordNet. Then EI = W/C.
Consistency (EC)Let I = 0. Let C be the number of classes and properties in ontology. Ci, if meaning in ontology is inconsistent, I+1. I = number of terms with inconsistent meaning. Ec = I/C.
Clarity (EA)Let Ci = name of class or property in ontology. Ci, count Ai , (the
number of word senses for that term in WordNet). Then EA = A/C.
Example Cont.: Pragmatic Quality (P) Refers to ontology’s usefulness for users or their
agents, irrespective of syntax or semantics. Accuracy
whether the claims an ontology makes are ‘true.’ Comprehensiveness
measure of the size of the ontology. Relevance
whether ontology satisfies the agent’s specific requirements.
Relevance (PR)
Pragmatic Quality (P) P = b1PO + b2PU + b3PR
Comprehensiveness (PO) Let C be the total number of classes and properties in ontology. Let V be the average value for C across entire library. Then PO = C/V.
Accuracy (PU)Let NS be the number of statements in ontology. Let F be the number
of false statements. PU = F/NS. Requires evaluation by domain expert
and/or truth maintenance system. Let NS be the number of statements in the ontology. Let S be the type
of syntax relevant to agent. Let R be the number of statements within NS that use S. PR = R / NS.
Example Cont.: Social Quality (O) Reflects that agents and ontologies exist
in communities. Authority
number of other ontologies that link to it History
number of times the ontology is accessed
Social Quality (O) O = b1OT + b2OH
Authority (OT) Let an ontology in the library be OA. Let the set of other ontologies
in the library be L. Let the total number of links from ontologies in L to OA be K. Let the average value for K across ontology library
be V. Then OT = K/V.
History (OH) Let the total number of accesses to an ontology be A. Let the average value for A across ontology library be H. Then OH = A/H.
References J. Brank, M. Groblnik and D. Meladenic,
“Ontology Evaluation”, SEKT Project Technical Report, 2003.