Equivalence class partitioning

Equivalence partitioning is a method for deriving test 
cases. In this method, classes of input conditions called 
equivalence classes are identified such that each member of 
the class causes the same kind of processing and output to 
occur. If the system were to handle one case in the class 
erroneously, it would handle all cases erroneously.

Equivalence partitioning drastically cuts down the number 
of test cases required to test a system reasonably.  It is 
an attempt to get a good 'hit rate', to find the most 
errors with the smallest number of test cases.