What happens at the extremes?
A rule that works for an ordinary case may fail at its boundaries. If a booking form allows groups of up to ten, test zero people, one person, ten people and eleven people. Boundary cases reveal missing definitions because they sit where the rule begins, ends or changes behaviour.
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Test where the rule changes
A booking system accepts groups of up to ten people. Testing a group of four confirms that one ordinary case works, but leaves several important questions unanswered.
- Is zero a valid booking?
- Does one person count as a group?
- Is ten included in the limit?
- What happens when someone enters eleven?
Boundary testing examines the edges of a rule's domain. The method applies beyond software. A refund policy, attendance requirement or queue procedure can also be unclear at a cutoff.
These cases help distinguish an intended restriction from an accidental omission. Testing every possible input may be impractical, but examining the smallest, largest and just-outside values often reveals where a definition needs to become more precise.