Software Testing 2nd Edition:Layout 1 10/1/10 4:56 PM Page 118
SOFTWARE TESTING
Exercise 4.10 Using Program Counting numbers from Exercise 4.9, what level of decision coverage would be achieved by the single input A = –1? The answer can be found at the end of the chapter. Other structure-based techniques More sophisticated techniques are available to provide increasingly complete code coverage. In some applications these are essential: for example, in a safety-critical system it is vital to know that nothing unacceptable happens at any point when the code is executed. Would you like to ‘fly by wire’ if you did not know what was happening in the software? The many well documented mishaps in computercontrolled systems provide compelling examples of what can happen if code – even code that is not providing essential functionality in some cases – does something unexpected. Measures such as condition coverage and multiple condition coverage are used to reduce the likelihood that code will behave in unpredictable ways by examining more of it in more complex scenarios. Coverage is also applicable to other types and levels of structure. For example, at the integration level it is useful to know what percentage of modules or interfaces has been exercised by a test suite, while at the functional level it is helpful to step through all the possible paths of a menu structure. We can also apply the idea of coverage to areas outside the computer, e.g. by exercising all the possible paths through a business process as testing scenarios. EXPERIENCE-BASED TECHNIQUES Experience-based techniques are those that you fall back on when there is no adequate specification from which to derive specification-based test cases or no time to run the full structured set of tests. They use the users’ and the testers’ experience to determine the most important areas of a system and to exercise these areas in ways that are both consistent with expected use (and abuse) and likely to be the sites of errors – this is where the experience comes in. Even when specifications are available it is worth supplementing the structured tests with some that you know by experience have found defects in other similar systems. Techniques range from the simplistic approach of ad hoc testing or error guessing through to the more sophisticated techniques such as exploratory testing, but all tap the knowledge and experience of the tester rather than systematically exploring a system against a written specification. Error guessing Error guessing is a very simple technique that takes advantage of a tester’s skill, intuition and experience with similar applications to identify special tests that may not be easy to capture by the more formal techniques. When applied after systematic techniques, error guessing can add another value in identifying
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