9  Ontologies

9.1 Learning objectives

Concepts

• Ontologies
• Classes
• Individuals
• Relationships
• Triples

Practical Skills

• Building a very basic ontology (individual assignment #2)

9.2 What are ontologies?

“An ontology encompasses a representation, formal naming and definition of the categories, properties and relations between the concepts, data and entities that substantiate one, many, or all domains of discourse.” (Wikipedia)

9.3 Components of an ontology

• Classes: The type of things that exist in the domain. They can be represented as nodes or as attributes of nodes in a graph.

• Individuals: These are instances of objects or concepts that belong to a class. They are represented as nodes in a graph.

• Relationships: These are the ways in which individuals and classes can be related to one another. They are represented as edges in the graph.

• Attributes: Attributes are characteristics of individuals, classes, or relationships (e.g. price of an individual of the class product, or date of birth of an individual of the class person).

• Triples: A triple is a representation of ontological components that take the following form:

SUBJECT PREDICATE OBJECT

For instance:

In a film ontology: Sylvester Stallone (subject) wrote (predicate) Rocky (object)

In a Dalhousie ontology: Philippe Mongeon (subject) teaches (predicate) Data Management (object)

In a hockey ontology: Brad Marchand (subject) plays for (predicate) Boston Bruins (subject)

Basically, the subject and the objects are two nodes in the network, and the predicate is the label of the edge that unites the nodes in the network.

9.4 Building a simple ontology

Let us now go through a series of five simple steps to build a very simple ontology. This is the exact process that I suggest that you follow for the final assignment. Please use this template for this purpose and for your final assignment. It comes pre-loaded with the example presented below.

Important note: in order to simplify the process we will not use attributes in this example, nor in the final assignment.

The five steps are:

1. Defining classes
2. Defining individuals
3. Assigning individuals to classes
4. Defining characteristics
5. Writing triples

Here’s an example of how to follow these steps to fill the provided template to build a basic pizza ontology.

1. What kind of things exist in the domain you want to represent?
   These are your classes. Make a list and insert them in the classes sheet of the template.

   Class_id	Class
   1	Pizza
   2	Base
   3	Sauce
   4	Cheese
   5	Topping

2. What individuals (instances of classes) do you want to include in my ontology?
   These will be the nodes in your network. Write in the template the name of each individual in the individuals sheet under the name column.
   
   

   Individual_id	Name
   1	Hawaiian
   2	Canadian
   3	Chicken Alfredo
   4	Alfredo
   5	Chicago style
   6	Mozzarella
   7	Pineapple

3. To which class do each individual belong?
   To what class does each of my individuals belong. Your individuals should belong to only one class. You should also have at least one individual for each class, otherwise that class is irrelevant for your ontology. Write the class to which each individual belongs in the class column of the individuals sheet.

   Individual_id	Name	Class_id
   1	Hawaiian	1
   2	Canadian	1
   3	Chicken Alfredo	1
   4	Tomato	3
   5	Chicago style	2
   6	Mozzarella	4
   7	Pineapple	5

4. What type of relationships between individuals exist in my ontology?
   In the relationships sheet, make a list of the different types of relationships that exist between your individuals. Your relationship should be verbs (e.g., acts_in, is_author_of, has_flavor, bought). For each relationship, consider whether it has an inverse relationship (e.g., has_actor, has_author, is_flavor_of, was_bought_by). If that is the case, indicate it in the inverse_relationship column. Here is an example:

   Relationship_id	Relationship	Inverse_relationship
   1	has_topping	is_topping_of
   2	has_base	is_base_of
   3	has_sauce	is_sauce_of
   4	has_cheese	is_cheese_of

   Important note: For your individual assignment you should have at least 3 relationships, excluding the inverse ones and you should not use the is_a relationships (these are used to link objects with their class, but for our purpose, we included the class as an attribute in the individuals sheet).

5. What relationships exist between individuals in my ontology?
   This is where you write your triples in the triples sheet of the template. Make sure that each individual has a relationship with at least one other. In the sheet you want to write the ID of each component, but it may be less confusing to start with the actual names of the individuals and relationship, like this:

   Subject_id	Relationship_id	Object_id
   Canadian	has_cheese	Mozzarella
   Canadian	has_base	Chicago style
   Hawaiian	has_topping	Pineapple
   Chicken Alfredo Pizza	has_sauce	Alfredo

   And then you can replace the values with their respective IDs.

   Subject_id	Relationship_id	Object_id
   2	1	6
   2	2	5
   1	1	7
   3	3	4

   As mentioned above, you can represent the relationship between an individual and its class in triples using the is_a relationship (e.g., Philippe is_a Professor). But for this assignment we will not use the is_a relationship, since the link between each individual and its class is already declared in the individuals table.

9.4.1 Visualizing our graph

I created a very primitive web application that can read data from the ontology template provided. Here is our graph:

9.5 Exercises

1. Download the template file that contains the data used in the example above ontology.
2. Create a list of 10 to 20 individuals that cover the five classes.
3. Create a list of 10 to 20 triples that cover the four types of relationships.
4. Visualize your ontology with this web application.
   Note: For the graph visualization tool to work, you must use the template and keep the names of the sheet and columns exactly as as they are.