Structured Testing - McCabe and Associates

A program with cyclomatic complexity 5 has the property that no set of 4 paths will suffice for
testing, even if, for example, there are 39 distinct tests that concentrate on the 4 paths. As discussed
in section 2, the cyclomatic complexity gives the number of paths in any basis set. This
means that if only 4 paths have been tested for the complexity 5 module, there must be, independent
of the programming language or the computational statements of the program, at least one
additional test path that can be executed. Also, once a fifth independent path is tested, any further
paths are in a fundamental sense redundant, in that a combination of 5 basis paths will generate
those further paths.
Notice that most programs with a loop have a potentially infinite number of paths, which are not
subject to exhaustive testing even in theory. The structured testing criterion establishes a complexity
number, v(G), of test paths that have two critical properties:
Several studies have shown that the distribution of run time over the statements in the program
has a peculiar shape. Typically, 50% of the run time within a program is concentrated within
only 4% of the code [KNUTH]. When the test data is derived from only a requirements point of
view and is not sensitive to the internal structure of the program, it likewise will spend most of
the run time testing a few statements over and over again. The testing criterion in this document
establishes a level of testing that is inherently related to the internal complexity of a program’s
logic. One of the effects of this is to distribute the test data over a larger number of independent
paths, which can provide more effective testing with fewer tests. For very simple programs
(complexity less than 5), other testing techniques seem likely to exercise a basis set of paths.
However, for more realistic complexity levels, other techniques are not likely to exercise a basis
set of paths. Explicitly satisfying the structured testing criterion will then yield a more rigorous
set of test data.
5.2 Intuition behind structured testing
The solid mathematical foundation of structured testing has many advantages [WATSON2].
First of all, since any basis set of paths covers all edges and nodes of the control flow graph, satisfying
the structured testing criterion automatically satisfies the weaker branch and statement
testing criteria. Technically, structured testing subsumes branch and statement coverage testing.
This means that any benefit in software reliability gained by statement and branch coverage testing
is automatically shared by structured testing.
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1. A test set of v(G) paths can be realized. (Again, see section 9.1 for discussion of the
more general case in which actual complexity is substituted for v(G).)
2. Testing beyond v(G) independent paths is redundantly exercising linear combinations
of basis paths.