Page 272 - DCAP305_PRINCIPLES_OF_SOFTWARE_ENGINEERING

Principles of Software Engineering

Notes Key Points
• Although in rare cases equivalence partitioning is also applied to outputs, typically it is
applied to the inputs of a component being tested.
• This method divides the input domain of a program into classes of data from which test
cases can be derived.
• It is based on an evaluation of equivalence classes for an input condition. An equivalence
class represents a set of valid or invalid states for input conditions. An input has certain
ranges which are valid and other ranges which are invalid.
• Supplemented by BVA(Boundary Value Analysis). Having determined the partitions of
possible inputs the method of boundary value analysis has to be applied to select the most
effective test cases out of these partitions.

13.4.6 Boundary Value Analysis
Boundary value analysis is a technique for test data assortment. A test engineer chooses
values that lie along data extremes. Boundary values include maximum, minimum, just inside
boundaries, just outside boundaries, typical values, and error values. The expectation is that, if
a systems works correctly for these extreme or special values, then it will work correctly for all
values in between. An effective way to test code is to exercise it at its natural boundaries. Boundary
Value Analysis is a method of testing that complements equivalence partitioning. In this case,
data input as well as data output are tested. The rationale behind BVA is that the errors typically
occur at the boundaries of the data. The boundaries refer to the upper limit and the lower limit
of a range of values or more commonly known as the “edges” of the boundary. BVA is a black
box approach to identifying test cases. In black box testing, test cases are selected based upon
the desired product functionality as documented in the specifications without consideration of
the actual internal structure of the program logic. A fundamental assumption in BVA is that
the majority of program errors will occur at critical Input (or Output) boundaries, places where
the mechanics of a calculation or data manipulation must change in order for the program to
produce a correct result. An example that illustrates the general concept of boundary values
would be a program that calculates income tax for a given income. In a progressive income
text scheme, the tax rate applied increases from low income brackets to high income brackets.
In this case, the critical input boundaries would be the set of incomes at which the applied tax
rate should change along with any minimum or maximum extremes of the income value. Thus,
the set of boundary incomes defines the limits of each tax bracket.
The BVA also serves as an introduction to other test techniques. Discussions of BVA in the
literature are often intermingled with a related black box technique known as Equivalence
Partitioning (EP), which utilizes the boundary values in an attempt to define partitions or
sets of test case that are equivalent in the sense that all test cases grouped within a particular
partition would reveal the presence of the same set of defects and likewise fail to detect other
defects. In its simplest form, once the partitions are identified, the set of test cases selected is
one representative test case from each partition. A distinct advantage of the EP technique is
that the total number of test cases is significantly smaller than the set of test cases identified
through BVA. In fact, the set of test cases identified by EP can be a subset of those identified by
BVA, and researchers have exploited this fact to reduce the total number of test cases identified
in merged BVA EP schemes.
Boundary value analysis
It is widely recognized that input values at the extreme ends of input domain cause more errors
in system. More application errors occur at the boundaries of input domain. ‘Boundary value
analysis’ testing technique is used to identify errors at boundaries rather than finding those exist
in centre of input domain. Boundary value analysis is a next part of Equivalence partitioning
for designing test cases where test cases are selected at the edges of the equivalence classes.

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