Equivalence partitioning (EP) is a blackbox testing technique. This technique is very common and mostly used by all the testers informally. Equivalence partitions are also known as equivalence classes.

As the name suggests Equivalence partitioning is to divide or to partition a set of test conditions into sets or groups that can be considered same by the software system.

As you all know that exhaustive testing of the software is not feasible task for complex software’s so by using equivalence partitioning technique we need to test only one condition from each partition because it is assumed that all the conditions in one partition will be treated in the same way by the software. If one condition works fine then all the conditions within that partition will work the same way and tester does not need to test other conditions or in other way if one condition fails in that partition then all other conditions will fail in that partition.

These conditions may not always be true however testers can use better partitions and also test some more conditions within those partitions to confirm that the selection of that partition is fine.

Lets take some examples:

A store in city offers different discounts depending on the purchases made by the individual. In order to test the software that calculates the discounts, we can identify the ranges of purchase values that earn the different discounts. For example, if a purchase is in the range of $1 up to $50 has no discounts, a purchase over $50 and up to $200 has a 5% discount, and purchases of $201 and up to $500 have a 10% discounts, and purchases of $501 and above have a 15% discounts.

Now we can identify 4 valid equivalence partitions and 1 invalid partition as shown below.

Invalid Partition Valid Partition(No Discounts) Valid Partition(5%) Valid Partition(10%) Valid Partition(15%)
$0.01 $1-$50 $51-$200 $201-$500 $501-Above