Cardinality describes how many of one Class is allowed against an instance of another Class.

In the NHS Data Model and Dictionary Cardinality is represented:

• In Diagrams by numbers on either end of the relationship line connecting Classes;
• On Classes by words describing the Relationship and Cardinality.

Cardinality represents the minimum and maximum number of instances of one Class against another.

In the NHS Data Model and Dictionary:

• 0..* means that a Class “may be related to one or more Class”
• 0..1 means that a Class “may be related to one and only one Class”
• 1..* means that a Class “must be related to one or more Class”
• 1 means that a Class “must be related to one and only one Class”

The minimum number is usually a 0 or a 1:

• 0 indicates a Class may be present and
• 1 indicates at least one instance of the Class must be present.

An example of Cardinality in diagrams is given below: