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Applying NFR Metrics: Reliability Requirement
publish date: 2026/06/06 23:08:33.409489 UTC
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A product manager says: 'The ticket-issuing system should be very reliable.' Which of the following rewrites this as a properly testable non-functional requirement?
Correct Answer
The ticket-issuing system shall have a mean time to failure (MTTF) of no less than 2,000 hours of continuous operation, with a maximum of 5 seconds of unplanned downtime per 24-hour operating period.
Explanation
A testable NFR uses measurable, verifiable metrics. MTTF ≥ 2,000 hours and ≤ 5 seconds downtime per day are both concrete, measurable thresholds that can be objectively tested. 'Very reliable', 'at all times', and 'as reliable as possible' are all goals — too vague to test objectively.
Reference
Software Engineering, Ian Sommerville, 9th edition