equivalence partition is a black box technique for writting 
test cases. 
 equivalence partition is process of reducing the hung
(infinite) set of possible into much smaller but still 
equally effective.
   In other word partition input condition which is tested, 
where each class representative(cover) a large set of other 
possible tests.
  In equivalence partion:-
   1) Range :- one valid         two invalid
   2) specific set:- one valid   one invalid.
   3) number:- one valid         two invalid

equivalence class partition is a subset of data that 
represents a larger class

EX: Credit card limit ($10,000 and $15000)
less than- 10,000 invalid
equal to ($10,000 and not as greater as 15000) valid

$15,000 or greater invalid

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.

1) If an input condition specifies a range, one valid and 
two invalid equivalence classes are defined. 

2) If an input condition requires a specific value, one 
valid and two invalid equivalence classes are defined. 

3) If an input condition specifies a member of a set, one 
valid and one invalid equivalence class are defined. 

4) If an input condition is boolean, one valid and one 
invalid class are defined.