What testing design techniques have you used for testing these functions?

For testing functions, especially business logic, the following test design techniques are often used:

• Equivalence Partitioning — dividing input data into classes of equivalence to test one representative from each class.
• Boundary Value Analysis — testing values at the edges of permissible ranges, as errors often occur there.
• Decision Table Testing — constructing decision tables to verify all combinations of conditions and corresponding actions.
• State Transition Testing — if a function depends on states, transitions between them are tested.

For example, if a function takes the client's age, classes can be distinguished as: less than 18, from 18 to 60, greater than 60, and boundary values 17, 18, 60, 61.