Equivalence Partitioning

Description: The idea behind the technique is to divide (i.e. to partition) a set of test conditions into groups or sets that can be considered the same (i.e. the system should handle them equivalently), hence 'equivalence partitioning'.Equivalence partitions are also known as equivalence classes - the two terms mean exactly the same thing.The equivalence-partitioning technique then requires that we need test only one condition from each partition. This is because we are assuming that all the conditions in one partition will be treated in the same way by the software. If one condition in a partition works, we assume all of the conditions in that partition will work, and so there is little point in testing any of these others. Conversely, if one of the conditions in a partition does not work, then we assume that none of the conditions in that partition will work so again there is little point in testing any more in that partition.

Example 1:

A saving account in a bank earns a different rate of interest depending on the balance in the account. In order to test the software that calculates the interest due, we can identify the ranges of balance values that earn the different rates of interest. If a balance in the range $0 up to $100 has a 3% interest rate, a balance over $100 and up to $1000 has a 5% interest rate, and balances of $1000 and over have a 7% interest rate, we would initially identify three valid equivalence partitions and one invalid partition as shown below.

Invalid Partition: -$0.01

Valid (for 3% interest): $0.00 to $100.00

Valid (for 5%) : $100.01 to $999.99

Valid (for 7%) : >$1000.00

So when designing the test cases for this software we would ensure that the three valid equivalence partitions are each covered once, and we would also test the invalid partition at least once. So for example, we might choose to calculate the interest on balances of-$10.00, $50.00, $260.00 and $1348.00

Example 2:
If you are testing for an input box accepting numbers from 1 to 1000 then there is no use in writing thousand test cases for all 1000 valid input numbers plus other test cases for invalid data. Using equivalence partitioning method above test cases can be divided into three sets of input data called as classes. Each test case is a representative of respective class. So in above example we can divide our test cases into three equivalence classes of some valid and invalid inputs.

Number of test cases:
a) One input data class with all valid inputs. Pick a single value from range 1 to 1000 as a valid test case. If you select other values between 1 and 1000 then result is going to be same. So one test case for valid input data should be sufficient.
b) Input data class with all values below lower limit. I.e. any value below 1, as a invalid input data test case.
c) Input data with any value greater than 1000 to represent third invalid input class.

So using equivalence partitioning you have categorized all possible test cases into three classes. Test cases with other values from any class should give you the same result. We have selected one representative from every input class to design our test cases. Test case values are selected in such a way that largest number of attributes of equivalence class can be exercised. Equivalence partitioning uses fewest test cases to cover maximum requirements.