Mean Time To Failure
Mean Time To Failure (MTTF) is a reliability metric that measures the average time until a non-repairable system or component fails.
“The average time from the start of operation to a failure for a component or system.”
(Definition reproduced from the ISTQB Glossary. Copyright belongs to ISTQB.)
View the complete ISTQB GlossaryWhat is Mean Time To Failure?
MTTF estimates the expected operational lifespan of components that are replaced rather than repaired after failure.
Lifetime estimate: Measures expected operating duration.
Non-repairable focus: Commonly used for consumable or replaceable components.
Reliability planning: Supports maintenance and replacement decisions.
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. Mean time to failure 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 Mean time to failure helps everyone discuss the problem using a shared testing vocabulary.
Practice Questions
Question 1
Which statement BEST describes Mean time to failure in the context of ISTQB terminology?
Question 2
A tester needs to explain Mean time to failure to a non‑technical stakeholder. Which approach is MOST appropriate?
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