Chapter 1 – Fundamentals of Testing
Defect Clustering – ISTQB Definition
Last Updated: July 12, 2026 · SoftwareTestPilot QA Team
Official ISTQB Definition
The principle that a small number of modules usually contain most of the defects discovered during pre-release testing, or are responsible for the most operational failures.
In simple words
Bugs are not spread evenly — a few risky modules host most of them, so focus extra testing there.
Exam tip
Defect clustering is one of the seven testing principles; it justifies risk-based test prioritization.
Related terms
- Seven Testing PrinciplesSeven quick truths every tester should internalize — from ‘you can’t test everything’ to ‘no bugs found doesn’t mean the product is good enough’.
- Pesticide ParadoxRunning the same tests forever stops catching new bugs. You need to review and refresh test cases regularly.
- DefectA flaw in the product — code, requirement, or design — that could cause a failure when executed.
Practice this term in the ISTQB Mock Test
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