Prototypes and requirements discovery go hand-in-hand. Measuring the success of a prototype is itself testing one or more requirements.

Expressing that measurement in code forms the basis of the first test cases that all future prototype enhancements must pass.

More test cases should be written to verify each of the discovered requirements:

• Test cases that the prototype passes will also be applied to future enhancements.
• Test cases that the prototype fails will define the next step in development.

Correct the prototype until each test case passes.

Stakeholders will add new requirements to a stable prototype. The prototype should not be modified unless it does not already support the new requirements. To be sure, new test cases should be written for each of the new requirements:

• Test cases that the prototype passes will also be applied to future enhancements.
• Test cases that the prototype fails will define the next step in development.

Correct the prototype until each test case passes.

Rinse and repeat.

The process described above applies Test-Driven Development (TDD) via iterative prototyping.

When TDD is applied to the nucleus prototype and its successors, it can easily be applied throughout the development process. TDD also leads to greater confidence in the integrity of the solution as it is developed, and thus also makes percent-complete and other project status measurements more meaningful.