Practically, due to time and budget considerations, it is not possible to perform exhausting testing for each set of test data, especially when there is a large pool test cases. We need an easy way or special techniques that can select test cases intelligent from the pool of test-case, such that all test scenarios are covered.

For that, we use two techniques - Equivalence Partitioning & Boundary Value Analysis testing techniques.

Equivalence Partitioning – It is a black box technique (code is not visible to tester) which can be applied to all levels of testing like unit, integration, system etc. In this technique, you divide set of test condition into partition that can be considered the same. To get a better idea let see an example.

Let's consider the behavior of tickets in the Flight reservation application, while booking a new flight.

Ticket values 1 to 10 are considered valid & ticket is booked. While value 11 to 99 are considered invalid for reservation and error message will appear, "Only ten tickets may be ordered at one time". 

Here is the test condition

1. Any Number greater than 10 entered in the reservation column (let say 11) is considered invalid.
2. Any Number less than 1 that is 0 or below, then it is considered invalid.
3. Numbers 1 to 10 are considered valid
4. Any 3 Digit Number say -100 is invalid.

We cannot test all the possible values because if done, number of test cases will be more than 100.To address this problem, we use equivalence partitioning hypothesis where we divide the possible values of tickets into groups or sets as shown below where the system behavior can be considered the same.

The divided sets are called Equivalence Partitions or Equivalence Classes. Then we pick only one value from each partition for testing. The hypothesis behind this technique is that if one condition/value in a partition passes all others will also pass. Likewise, if one condition in a partition fails, all other conditions in that partition will fail.

Boundary Value Analysis- in Boundary Value Analysis, you test boundaries between equivalence partitions

In our earlier example instead of checking, one value for each partition you will check the values at the partitions like 0, 1, 10, 11 and so on. As you may observe, you test values at both valid and invalid boundaries. Boundary Value Analysis is also called range checking.

Equivalence partitioning and boundary value analysis are closely related and can be used together at all levels of testing.