A Comparison of Three Systems Modeling Methodologies - High-Performance Web Databases: Design, Development, and Deployment - page 196

Model has been described as a thinking tool that uses graphical notation
for describing and defining the vocabulary and conceptualization of a
problem. It identifies what a problem is about and organizes these ideas
into a format structure. This model can be regarded as a variation of the ER
model discussed in the previous section.
The basic elements of this model are the object, its attributes, and the rela-
tionships between objects. The object is the basic part of the system. Shlaer
and Mellor define an object as an abstraction of a set of real-world items such
that all of the real-world items in the set-the instances-have the same charac-
teristics and all instances are subject to and conform to the same rules.
An object is depicted as a rectangle. The name is placed at the top of the
rectangle (see Exhibit 4). To identify objects, the Information Model uses
five different categories-tangible things, roles, incidents, interactions and
specifications.
A role is the part played by a person or organization. Examples of role
objects include a doctor, a patient, a nurse, a department, an employee,
and a supervisor. Sometimes role objects distinguish between different
roles played by the same person. An example of this is an employee who is
also a supervisor.
Another type of object is an incident. They are used to represent an
occurrence or event-something which happens at a specific time. Examples
of incident objects are a flight, an accident, or a service call.
The fourth type of object is the interaction object. Interaction objects
can be compared to transactions or contracts. An example is a purchase
which relates a buyer, a seller, and the purchase item.
The last type of object is a specification object. Specification objects fre-
quently show up in inventory or manufacturing applications. An example
of this type of object is a radio specification. This object has the specifica-
tions of the specific model. There would most likely be another object
which describes each instance of a specific model of a radio.
The Shlaer and Mellor method specifies an object description, naming
conventions, and methods to test if objects are valid. An object description
is a short, informative statement that allows an analyst to tell with cer-
tainty whether a particular item is an instance of the object conceptualized
in the information model.
Attributes are another important part of the Information Model. An
attribute is defined as the abstraction of a single characteristic possessed
by all the entities that were, themselves, abstracted as an object. An
attribute is represented by a line item inside the object rectangle. The
objective is a collection of attributes that are complete, fully factored, and
mutually independent.