Ontologies
Angela Maduko
Acknowledgements
 Ontology Development 101: A Guide to
Creating Your First Ontology
 Borrowed slides from
– http://www.ifnet.it/elag2002/papers/pap9.ppt
– http://www.ccs.neu.edu/home/kenb/pub/2003
/02/public.ppt
Outline
 Background on ontologies
 Ontology development
–
–
–
–
Requirements and Analysis
Design and Implementation
Testing and Validation
Maintenance
What is an ontology?
Most of available definitions
are of little help to the layman:
“An ontology is an explicit specification of a conceptualization”
Thomas R. Gruber
“An ontology is a logical theory accounting for the intended
meaning of a formal vocabulary”
Nicola Guarino
“A theory concerning the kinds of entities and specifically the kinds
of abstract entities that are to be admitted to a language system”
Webster’s Third New International Dictionary
What is an Ontology?
Some definitions sound
a bit more familiar:
“An ontology is a formal explicit description of concepts in a
domain of discourse (classes […]), properties of each concept […]
(slots […]), and restrictions on slots (facets […])”
Natalya F. Noy & Deborah L. McGuinness
“An ontology is a formal definition of a body of knowledge”
James Hendler
“An ontology is a catalog of the types of things that are assumed to
exist in a domain of interest D from the perspective of a person who
uses a language L for the purpose of talking about D”
John F. Sowa
What general features can we draw from all
of these definitions?
 A given field/domain of interest within the
boundaries of which “things” are assumed
to exist, to “be” and about which we wish
to “talk” in a formalized, explicit, and
semantically consistent way
To talk about what is:
Ontologies
 Defines a common vocabulary for
researchers who need to share
information in a domain.
 It includes machine-interpretable
definitions of basic concepts in the domain
and relations among them.
Ontologies
 Ontology: What exists in a domain and
how they relate with each other.
 Formal ontology: Formal treatment of the
concepts and relationships in a domain.
 Simple Example:
– Employees work for Companies
– Employees report to Employees
Statements
George is an employee.
George works for Sony.
George reports to Adam.
Fred works for a company.
An object is an instance of
the Employee class.
An object in the Employee class is linked
with an object in the Company class
via the works_for relationship.
An object in the Employee class is linked
with another object in the same class
via the reports_to relationship.
Fred reports to two other employees.
Fred must report to another employee.
George says that
Fred works for Toyota.
Purposes of Ontologies


Basis for communication
– Between people (may be informal)
– Between agents (formal ontologies)
Some Applications
– Representing and storing data (e.g., DB schema)
– Knowledge sharing within and between domains
– Search and retrieval
– Classification and organization of data resources
Logic: Open versus Closed
 Open (monotonic) logic gives
different answers to queries than a
closed logic.
works for
Em ployee
0..*
Com pany
1
 Suppose that Fred is just an
employee:
– Closed world: violates constraint
– Open world: Fred works for a company,
but the company is not known.
Logic: Open versus Closed
works for
Em ployee
0..*
Com pany
1
 Suppose that Fred works for both
Sony and IBM:
– Closed world: violates constraint
– Open world: Sony and IBM are the same
company!
Outline
 Background on ontologies
 Ontology development
–
–
–
–
Requirements and Analysis
Design and Implementation
Testing and Validation
Maintenance
Why Develop an Ontology
 To share common understanding of the
structure of information among people or
software agents
 To enable reuse of domain knowledge
 To make domain assumptions explicit
 To separate domain knowledge from the
operational knowledge
 To analyze domain knowledge
Ontology Development
 Phases of Ontology Development
– Requirements and Analysis
 Determine domain and scope of ontology
 What is the domain that the ontology will cover?
 For what are we going to use the ontology?
 To what types of questions will the information in the
ontology provide answers?
– Design and Implementation
– Testing and Validation
– Maintenance
 Who will use and maintain the ontology?
Requirements and Analysis
 Least understood of all phases
 Direct involvement by stakeholders is
essential, but how?
 Specifying the scope is important, but not
all languages support it.
 Point of view is also relevant.
 These phases offer significant
opportunities for new methodologies and
processes.
Ontology Design
 Tasks involved in designing an ontology
include
– Defining classes in the ontology
– Arranging the classes in a taxonomic
(subclass–superclass) hierarchy
– Defining slots (relationships) and describing
allowed values for these slots
– Filling in the values for slots for instances.
Fundamental Rules in Ontology Design
 There is no one correct way to model a domain—




there are always viable alternatives.
The best solution almost always depends on the
application that you have in mind and the
extensions that you anticipate.
Ontology development is necessarily an iterative
process.
Concepts in the ontology should be close to
objects (physical or logical) and relationships in
your domain of interest.
These are most likely to be nouns (objects) or
verbs (relationships) in sentences that describe
your domain.
Example
 Which wine characteristics should I consider






when choosing a wine?
Is Bordeaux a red or white wine?
Does Cabernet Sauvignon go well with seafood?
What is the best choice of wine for grilled meat?
Which characteristics of a wine affect its
appropriateness for a dish?
Does a bouquet or body of a specific wine change
with vintage year?
What were good vintages for Napa Zinfandel?
Step 3. Enumerate important terms in the
ontology
 What are the terms we would like to talk about?
 What properties do those terms have?
 What would we like to say about those terms?
 Example –
–
–
–
wine, grape, winery, location,
a wine’s color, body, flavor and sugar content;
different types of food, such as fish and red meat;
subtypes of wine such as white wine, and so
Step 4. Define the classes and the class
hierarchy
 Methods
– Top-down
– Bottom-up
– Combination
Step 4. Define the classes and the class
hierarchy
 Hierarchical arrangements of concepts
 If a class A is a superclass of class B, then every
instance of B is also an instance of A
 This implies “In other words, the class B
represents a concept that is a “kind of” A.”
Step 4. Define the classes and the class
hierarchy
 Top-down
– Food and Wine -- followed by White, Blush and
Red
 Bottom-up
– define specific wine class first and then work
your way up
 Combination
Step 5. Define the properties of classes—
slots
 Types of properties
– “intrinsic” properties such as the flavor of a wine;
– “extrinsic” properties such as a wine’s name, and area it
comes from;
– parts, if the object is structured; these can be both
physical and abstract “parts” (e.g., the courses of a
meal)
– relationships to other individuals; these are the
relationships between individual members of the class
and other items (e.g., the maker of a wine, representing a
relationship between a wine and a winery, and the grape the
wine is made from.)
Step 5. Define the properties of classes—
slots
 Examples
– a wine’s
 color,
 body,
 flavor
 sugar content
– location of a winery.
Step 6. Define the facets of the slots
 Slot cardinality
– defines how many values a slot can have.
 Slot-value type
– what types of values can fill in the slot.
– common value types:
 String
 Number
 Boolean
 Enumerated
Step 6. Define the facets of the slots
 value type
 allowed values
 the number of the values (cardinality)
 other features of the values the slot can take.
Step 6 - rules slot Domain and Ranges



When defining a domain or a range for a slot, find the
most general classes or class that can be respectively
the domain or the range for the slots .
On the other hand, do not define a domain and range
that is overly general: all the classes in the domain of
a slot should be described by the slot and instances of
all the classes in the range of a slot should be
potential fillers for the slot.
Do not choose an overly general class for range (i.e.,
one would not want to make the range THING) but
one would want to choose a class that will cover all
fillers
Step 6 - rules slot Domain and Ranges
 If a list of classes defining a range or a
domain of a slot
– includes a class and its subclass, remove the
subclass.
– contains all subclasses of a class A, but not the
class A itself, the range should contain only the
class A and not the subclasses.
– contains all but a few subclasses of a class A,
consider if the class A would make a more
appropriate range definition.
Step 7 - Creating Instances
 Body: Light
 Color: Red
 Flavor: Delicate
 Tannin level: Low
 Grape: Gamay (instance of the Wine grape class)
 Maker: Chateau-Morgon (instance of the Winery class)
 Region: Beaujolais (instance of the Wine-Region class)
 Sugar: Dry
Step 8 - Defining classes and a class
hierarchy
 Ensuring that the class hierarchy is correct
– An “is-a” relation
 A subclass of a class represents a concept that is a “kind of”
the concept that the superclass represents.
– A single wine is not a subclass of all wines
– Transitivity of the hierarchical relations
Step 8 - Defining classes and a class
hierarchy
 Evolution of a class hierarchy
– distinction between classes and their names
– hence synonyms of concept name do not represent
different classes
– Avoid class hierarchy cycles
– All the siblings in the hierarchy (except for the ones at
the root) must be at the same level of generality.
Step 8 - Defining classes and a class
hierarchy
 How many is too many and how few are too
few?
– If a class has only one direct subclass there may be a
modeling problem or the ontology is not complete.
– If there are more than a dozen subclasses for a given
class then additional intermediate categories may be
necessary. (may not always be possible)
Step 8 - Defining classes and a class
hierarchy
 Multiple Inheritance
 When do you introduce a new class
– Subclasses of a class usually
 (1) have additional properties that the superclass does not
have, or
 (2) restrictions different from those of the superclass, or
 (3) participate in different relationships than the superclasses
Step 8 - Defining classes and a class
hierarchy
 A new class or a property value?

– Class White Wine or simply property of class
Wine that takes value White
– Depends on how important a concept White
Wine is in the domain
An instance or a class?
– starts with deciding what is the lowest level of
granularity in the representation.
– Individual instances are the most specific concepts
represented in a knowledge base.
Step 8 - Limiting the scope
– The ontology should not contain all the possible
information about the domain: you do not need to
specialize (or generalize) more than you need for
your application (at most one extra level each way).
– The ontology should not contain all the possible
properties of and distinctions among classes in the
hierarchy.
Design and Implementation: Patterns and Reuse




Reuse existing ontologies if they exist.
Transform another ontology
Alignment: inter-ontological relationships
Composition: Transform and combine many smaller
ontologies
Combined Ontology
Ontology1
Ontology2
Common Features
Design and Implementation: Refactoring
 Move methods from one class to another
(for example, from subclasses to
superclasses).
 Reification and unreification: changing
relationships into classes or vice versa.
 Refactoring is useful at the design,
implementation and maintenance phases.
Example of Refactoring
Example of Reification
Testing and Validation


Validation of requirements
Consistency checking
– incompatible domain and
range definitions for
transitive, symmetric, or
inverse properties
– cardinality properties
– requirements on property
values can conflict with
domain and range
restrictions
In order to better understand what
an ontology is, let’s make one.
What is the specific goal of our ontology?
 To





describe the process of making tea, in order to
have it automated?
To evaluate the quality of a specific cup of tea?
To evaluate the skills of an apprentice tea-maker?
To choose the best type of cup of tea according to
time and meal?
To allow tea-houses to share differently structured
data?
&c…
The scope of an ontology must be precisely defined
What is required in order to make a cup of
tea?
 Ingredients: tea, water, milk or lemon, sugar…
 Utensils: kettle, tea-pot, tea strainer, cup, spoon…
 Actions: boiling the water, pouring the boiling water


into the tea-pot, pouring the water out of the teapot, simmering some fresh water…
Measurements: temperature of water, duration of
the infusing process…
&c…
All of these are the “things” assumed to exist in our
domain of interest: “the making of a cup of tea”
Is there a hierarchy among these “things”?
Ingredients
Utensils
Actions
Measurements
Boiling
Duration of
Required
Optional
Tea-pot
Temperature
Kettle
the water
the infusing
ingredients ingredients
etc.
of water
etc.
process
Tea
Water
Milk
Lemon
Sugar
Are we sure all of us understand these
“things” the same way?
 What is an “ingredient”?
Please define: …
 What is a “required ingredient”?
Please define: …
 What is an “optional ingredient”?
Please define: …
How can we describe each of these
“things”?
origin: beet? maple? cane?
Sugar
colour: brown? white?
form:
caster? lumps? liquid?
candy? granulated?
…
What about fructose and Aspartam?
Let’s create new classes and sub-classes:
Sweeteners
Natural
sweeteners
Sugar origin: …
colour: …
form: …
Diet
sweeteners
Fructose
Artificial
sweeteners
Aspartam
The development of an ontology is an iterative process:
an ontology needs to be evaluated, revised, evaluated,
revised, evaluated…
Do we really need all these refinements
about sweeteners?
 Not necessarily: depends on our purposes
 Perhaps we might be just as happy with:
Sweeteners
type: cane sugar, beet sugar, fructose, Aspartam…
form: lumps, granulated…
“weightwatchingcally correct?”: Yes or No
“There is no one correct way to model a domain
— there are always viable alternatives”
(N. F. Noy & D. L. McGuinness)
How are these “things” interrelated?
Water
is passive participant in  Boiling
 has passive participant the water
has as tool 
 is tool of
Kettle
Are we sure all of us understand
these relationships the same way?
• What does “is passive participant in / has passive participant” mean?
Please define: …
• What does “has as tool / is tool of” mean?
Please define: …
Did you know you knew so many things about
a simple cup of tea?
Ontologies are related to the
concept of knowledge representation