Boundary value analysis
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Boundary value analysis is a software testing design technique to determine test cases covering off-by-one errors. The boundaries of software component input ranges are areas of frequent problems.
[edit] Introduction
Testing experience has shown that especially the boundaries of input ranges to a software component are liable to defects. A programmer implement e.g. the range 1 to 12 at an input, which e.g. stands for the month January to December in a date, has in his code a line checking for this range. This may look like:
if (month > 0 && month < 13)
But a common programming error may check a wrong range e.g. starting the range at 0 by writing:
if (month >= 0 && month < 13)
For more complex range checks in a program this may be a problem which is not so easily spotted as in the above simple example.
[edit] Applying boundary value analysis
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To set up boundary value analysis test cases the tester first has to determine which boundaries are at the interface of a software component. This has to be done by applying the equivalence partitioning technique. Boundary value analysis and equivalence partitioning are inevitably linked together. For the example of the month a date would have the following partitions:
... -2 -1 0 1 .............. 12 13 14 15 .....
--------------|-------------------|---------------------
invalid partition 1 valid partition invalid partition 2
Applying boundary value analysis a test case at each side of the boundary between two partitions has to be selected. In the above example this would be 0 and 1 for the lower boundary as well as 12 and 13 for the upper boundary. Each of these pairs consists of a "clean" and a "dirty" test case. A "clean" test case should gives a valid operation result of the program. A "dirty" test case should lead to a correct and specified input error treatment such as the limiting of values, the usage of a substitute value, or in case of a program with a user interface, it has to lead to warning and request to enter correct data. The boundary value analysis can have 6 test cases: n, n-1, and n+1 for the upper limit; and n, n-1, and n+1 for the lower limit.
A further set of boundaries has to be considered when test cases are set up. A solid testing strategy also has to consider the natural boundaries of the data types used in the program. If working with signed values, for example, this may be the range around zero (-1, 0, +1). Similar to the typical range check faults, there tend to be weaknesses in programs in this range. e.g. this could be a division by zero problem where a zero value may occur although the programmer always thought the range started at 1. It could be a sign problem when a value turns out to be negative in some rare cases, although the programmer always expected it to be positive. Even if this critical natural boundary is clearly within an equivalence partition it should lead to additional test cases checking the range around zero. A further natural boundary is the natural lower and upper limit of the data type itself. E.g. an unsigned 8-bit value has the range of 0 to 255. A good test strategy would also check how the program reacts at an input of -1 and 0 as well as 255 and 256.
The tendency is to relate boundary value analysis more to so called black box testing, which is strictly checking a software component at its interfaces, without consideration of internal structures of the software. But looking closer at the subject, there are cases where it applies also to white box testing.
After determining the necessary test cases with equivalence partitioning and subsequent boundary value analysis, it is necessary to define the combinations of the test cases when there are multiple inputs to a software component.
