Equivalence partitioning, generated

Equivalence partitioning, also called equivalence class partitioning, divides the possible values of an input into groups the system should treat the same way. If an age field accepts 18 to 60, then 25, 37 and 52 all take the same path through the code. Testing one of them tells you about all of them, so one value per partition is enough.

The partitions include the invalid ones: ages below 18, ages above 60, decimals where whole numbers are required, text, and an empty field. Each invalid partition is a distinct way for input to be wrong, and each deserves its own test.

Valid partitions from different inputs can be combined in one test, because a successful submission proves each value was accepted. Invalid partitions cannot. If a form is submitted with both an invalid age and an invalid email, and it is rejected, you have no idea which check fired. The other check might not exist at all.

This is called defect masking. The rule that follows from it is simple: every invalid partition gets a test in which it is the only thing wrong. That is why the generator lists invalid classes individually instead of combining them.

The representative of a partition should sit well inside it, not on its edge. For 18 to 60, choose something like 39, not 18. A test on the boundary belongs to boundary value analysis, and if it fails you cannot tell whether the partition or its edge is wrong. Keeping the two techniques apart keeps failures easy to diagnose.

For invalid partitions, choose a value that only the rule being tested can reject. A decimal such as 39.5 is inside the valid range, so if it is rejected, the whole-number rule did it. A text value with a digit in it, at a valid length, isolates the character rule the same way.

An optional field and a required field have different partitions for the empty value: valid in one, invalid in the other. A case-insensitive list of values makes "express" valid when "Express" is allowed; a case-sensitive one makes it invalid. Whitespace is another hidden partition: a value of three spaces looks filled in but should be treated as empty.

When the requirement does not say which way a detail goes, the generator still lists the partition and its note says what to decide. An unanswered question about whether input is trimmed is a requirement gap, and a requirement gap is cheaper to fix before the code is written.