2.3Equivalence Partitioning (EP)

Divide inputs into groups (partitions) where all values in a partition are expected to behave the same. Test one representative value from each partition.

The Principle

If the system treats all values in a range identically, testing one value is sufficient to represent the entire range. This dramatically reduces test count while maintaining meaningful coverage.

Example: Same Age Field (18-65)

Partition	Range	Representative Value	Expected
Invalid low	< 18	10	Rejected
Valid	18-65	30	Accepted
Invalid high	> 65	70	Rejected

EP for Non-Numeric Inputs

Equivalence partitioning works for any input type:

Input	Valid Partitions	Invalid Partitions
Email field	Valid email format (user@domain.com)	Missing @, missing domain, empty string, spaces only
File upload	Allowed formats (.jpg, .png, .pdf)	Disallowed formats (.exe, .bat), empty file, oversized file
Country dropdown	Countries in the list	(cannot select invalid, but test empty selection)
Search query	Alphanumeric strings	SQL injection attempts, XSS payloads, extremely long strings

Combining EP and BVA

EP reduces test count. BVA sharpens the selection within partitions. Use them together:

1. First, identify partitions with EP
2. Then, within each partition boundary, apply BVA

For the age field: EP gives you three partitions. BVA refines the boundary between partitions to specific values (17, 18, 65, 66).