Mean Time Between Failures
Mean Time Between Failures (MTBF) is a reliability metric that measures the average operating time between one system failure and the next for repairable systems.
“The average time between consecutive failures of a component or system.”
(Definition reproduced from the ISTQB Glossary. Copyright belongs to ISTQB.)
View the complete ISTQB GlossaryWhat is Mean Time Between Failures?
MTBF helps organizations evaluate reliability by estimating how long a system typically operates before another failure occurs.
Reliability measure: Higher MTBF generally indicates greater reliability.
Repairable systems: MTBF assumes the system is repaired after each failure.
Trend analysis: Used to compare reliability across releases or products.
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 between failures 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 between failures helps everyone discuss the problem using a shared testing vocabulary.
Practice Questions
Question 1
Which statement BEST describes Mean time between failures in the context of ISTQB terminology?
Question 2
A tester needs to explain Mean time between failures to a non‑technical stakeholder. Which approach is MOST appropriate?
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