For Example, 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 the Equivalence Partitioning method above test cases can be divided into three sets of input data called classes. Each test case is representative of a respective class.
So in the above example, we can divide our test cases into three equivalence classes of some valid and invalid inputs.
Test cases for input box accepting numbers between 1 and 1000 using Equivalence Partitioning:
#1) 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 the result is going to be the same. So one test case for valid input data should be sufficient.
#2) Input data class with all values below the lower limit. I.e. any value below 1, as an invalid input data test case.
#3) Input data with any value greater than 1000 to represent the 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.
Boundary Value Analysis
It's widely recognized that input values at the extreme ends of the input domain cause more errors in the system. More application errors occur at the boundaries of the input domain. ‘Boundary Value Analysis' Testing technique is used to identify errors at boundaries rather than finding those that exist in the center of the input domain.
Boundary Value Analysis is the next part of Equivalence Partitioning for designing test cases where test cases are selected at the edges of the equivalence classes.
Test cases for input box accepting numbers between 1 and 1000 using Boundary value analysis:
#1) Test cases with test data exactly as the input boundaries of input domain i.e. values 1 and 1000 in our case.
#2) Test data with values just below the extreme edges of input domains i.e. values 0 and 999.
#3) Test data with values just above the extreme edges of the input domain i.e. values 2 and 1001.
Boundary Value Analysis is often called as a part of the Stress and Negative Testing.
Note: There is no hard-and-fast rule to test only one value from each equivalence class you created for input domains. You can select multiple valid and invalid values from each equivalence class according to your needs and previous judgments.
For Example, if you divided 1 to 1000 input values invalid data equivalence class, then you can select test case values like 1, 11, 100, 950, etc. Same case for other test cases having invalid data classes.
This should be a very basic and simple example to understand the Boundary Value Analysis and Equivalence Partitioning concept.