Could You Prove It Works? Writing Requirements That Pass the Test

Start with something you can check

Imagine you are helping to build a drone. Someone writes: “The drone must warn the pilot quickly when its battery is low.”

That sounds sensible. But what counts as “quickly”? And how low is “low”?

A requirement is a written statement of what a system must do or a limit it must meet. A testable requirement gives engineers enough detail to decide whether the result passes or fails. It should not depend on who watches the test.

Replace vague words with clear limits

For a classroom prototype, you might write:

“The controller shall display a low-battery warning within two seconds after the drone’s reported battery charge falls below 20%, while the drone is connected to the controller.”

Here, “shall” marks something the design must do. The numbers are example targets, not recommendations for a real aircraft. Real limits need evidence, such as battery behaviour and the time a pilot needs to respond.

The revised requirement identifies:

• What acts: the controller.
• What happens: a low-battery warning appears.
• What triggers it: reported charge falls below 20%.
• What limit applies: the warning appears within two seconds.
• What condition matters: the drone is connected.

Plan the test before building

Now imagine checking that requirement. A test setup could send the controller a simulated battery reading, meaning a reading created for the test rather than taken from a battery.

Change the reading from above 20% to below it. Record when the controller receives it and when the warning appears. Compare the elapsed time with the two-second limit.

This reveals another question: does timing start when the drone reports the change or when the controller receives it? Agree that detail and update the requirement before testing.

Keep separate behaviours in separate requirements. What happens if the connection fails needs its own rule.

Engineer-approved AI can help spot vague wording and missing test conditions, while engineers make the decisions.