Defect Density
Defect density is a quality metric that measures the number of defects relative to the size of a software product.
“The number of defects per unit size of a work product.”
(Definition reproduced from the ISTQB Glossary. Copyright belongs to ISTQB.)
View the complete ISTQB GlossaryWhat is Defect Density?
Organizations use defect density to evaluate software quality, compare components, and identify areas that may require additional testing or improvement.
Measurement: Commonly expressed as defects per thousand lines of code or function points.
Comparison: Helps compare quality across modules or releases.
Trend analysis: Changes over time indicate quality improvement or deterioration.
Real World Example
A hospital medication system shows a different dosage warning in test than the one specified by the clinical safety rule.
The team needs precise language to separate what was observed, what should have happened, and whether the difference represents a reportable issue. Defect density provides that distinction.
The tester records the steps, data, observed behavior, expected behavior, and evidence so triage can focus on patient safety impact instead of vague descriptions.
The issue is handled faster because Defect density helps everyone discuss the problem using a shared testing vocabulary.
Practice Questions
Question 1
Which statement BEST describes Defect density in the context of ISTQB terminology?
Question 2
A tester needs to explain Defect density to a non‑technical stakeholder. Which approach is MOST appropriate?
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