Can you give an example of equivalence classes?

Equivalence classes are groups of input data for which the system behaves the same way. For example, for a "age" field with an allowable range from 18 to 65, the classes can be distinguished as:

• Valid class: 18–65
• Invalid class: less than 18
• Invalid class: greater than 65

Testing one value from each class allows for effective testing of the system's behavior without an excessive number of tests.