Boundary value analysis is a software testing technique in which tests are designed to include representatives of boundary values. Values on the minimum and maximum edges of an equivalence partition are tested. The values could be either input or output ranges of a software component. Since these boundaries are common locations for errors that result in software faults they are frequently exercised in test cases.
The expected input and output values to the software component should be extracted from the component specification. The values are then grouped into sets with identifiable boundaries. Each set, or partition, contains values that are expected to be processed by the component in the same way. Partitioning of test data ranges is explained in the equivalence partitioning test case design technique. It is important to consider both valid and invalid partitions when designing test cases.
For an example, if the input values were months of the year, expressed as integers, the input parameter 'month' might have the following partitions:
... -2 -1 0 1 .............. 12 13 14 15 .....
--------------|-------------------|-------------------
invalid partition 1 valid partition invalid partition 2
The boundary between two partitions is the place where the behavior of the application changes and is not a real number itself. The boundary value is the minimum (or maximum) value that is at the boundary. The number 0 is the maximum number in the first partition, the number 1 is the minimum value in the second partition, both are boundary values. Test cases should be created to generate inputs or outputs that will fall on and to either side of each boundary, which results in two cases per boundary. The test cases on each side of a boundary should be in the smallest increment possible for the component under test, for an integer this is 1, but the input was a decimal with 2 places then it would be .01. In the example above there are boundary values at 0,1 and 12,13 and each should be tested.
Boundary value analysis does not require invalid partitions. Take an example where a heater is turned on if the temperature is 10 degrees or colder. There are two partitions (temperature<=10, temperature>10) and two boundary values to be tested (temperature=10, temperature=11).
Where a boundary value falls within the invalid partition the test case is designed to ensure the software component handles the value in a controlled manner. Boundary value analysis can be used throughout the testing cycle and is equally applicable at all testing phases.