Chapter 4: Defect based test designing techniques

Defect based test designing techniques

According to ISTQB, A defect-based test design technique is one in which the type of defect sought is used as the basis for test design, with tests derived systematically from what is known about the type of defect. Unlike specification-based testing which derives its tests from the specification, defect-based testing derives tests from defect taxonomies (i.e., categorized lists) that may be completely independent from the software being tested. The taxonomies can include

•     lists of defect types
•     root causes
•     failure symptoms
•     and other defect-related data.

Defect-based testing may also use lists of identified risks and risk scenarios as a basis for targeting testing. This test technique allows the tester to target a specific type of defect or to work systematically through a defect taxonomy of known and common defects of a particular type.

The Test Analyst uses the taxonomy data to determine the goal of the testing, which is to find a specific type of defect. From this information, the Test Analyst will create the test cases and test conditions that will cause the defect to manifest itself, if it exists.

Defect-based testing can be applied at any testing level but is most commonly applied during system testing. There are standard taxonomies that apply to multiple types of software. This non-product specific type of testing helps to leverage industry standard knowledge to derive the particular tests. By adhering to industry-specific taxonomies, metrics regarding defect occurrence can be tracked across projects and even across organizations.

Multiple defect taxonomies exist and may be focused on particular types of testing, such as usability. It is important to pick a taxonomy that is applicable to the software being tested, if any are available. For example, there may not be any taxonomies available for innovative software. Some organizations have compiled their own taxonomies of likely or frequently seen defects. Whatever taxonomy is used, it is important to define the expected coverage prior to starting the testing.

Coverage The technique provides coverage criteria which are used to determine when all the useful test cases have been identified. As a practical matter, the coverage criteria for defect-based techniques tend to be less systematic than for specification-based techniques in that only general rules for coverage are given and the specific decision about what constitutes the limit of useful coverage is discretionary.

As with other techniques, the coverage criteria do not mean that the entire set of tests is complete, but rather that defects being considered no longer suggest any useful tests based on that technique.

Types of Defects The types of defects discovered usually depend on the taxonomy in use. If a user interface taxonomy is used, the majority of the discovered defects would likely be user interface related, but other defects can be discovered as a byproduct of the specific testing.

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