Pairwise Testing
Pairwise testing generates a minimal set of test cases that covers every possible pair of input parameter values at least once, based on the observation that most defects are triggered by interactions between just two parameters. It's the most common practical form of combinatorial testing.
For a system with 5 parameters each having 4 possible values, exhaustive testing would require over a thousand test cases; pairwise testing typically reduces that to a few dozen while still guaranteeing every two-parameter combination is exercised at least once — a dramatic reduction in effort for a well-documented, small drop in theoretical coverage.
Free tools (PICT from Microsoft is widely used) generate the actual reduced test case list automatically from a simple description of the parameters and their values, making pairwise testing practical to apply without manual combinatorics.
