An equivalence class is a subset of data that is 
representative of a larger class. Equivalence partitioning 
is a technique for testing equivalence classes rather than 
undertaking exhaustive testing of each value of the larger 
class. For example, a program which edits credit limits 
within a given range (1,000 - 1,500) would have three 
equivalence classes:

< 1,000 (invalid)
Between 1,000 and 1,500 (valid)
> 1,500 (invalid)