Equivalence Partitioning
Equivalence partitioning divides an input range into groups (partitions) that should all behave the same way, then tests just one representative value from each group instead of every possible value. It's the foundational technique that makes exhaustive input testing unnecessary in most cases.
For a field accepting ages 18-65, equivalence partitioning identifies three partitions: invalid-too-low (below 18), valid (18-65), and invalid-too-high (above 65). Testing one value from each — say 10, 30, and 80 — gives confidence about the whole range without testing all 100+ individual possible ages.
It pairs directly with boundary value analysis: partitioning tells you which classes of input exist, and boundary value analysis tells you the partition edges are exactly where to add extra scrutiny, since that's where off-by-one bugs actually cluster.
