What is the meaning of MTTR and how does it differ from MTBF?

• A. MTTR is Mean Time To Repair; MTBF is Mean Time Between Failures.
• B. MTTR is Minimum Time To Restore; MTBF is Maximum Time Before Failure.
• C. MTTR is Mean Time To Repair; MTBF is Mean Time To Fail.
• D. MTTR is Mean Time To Response; MTBF is Mean Time Between Failures.

Answer: (B)

MTTR and MTBF measure different aspects of reliability: repair time versus time between failures. MTTR stands for Mean Time To Repair (often called Mean Time To Restore); it is the average time required to diagnose and fix a failed component and bring the system back online. MTBF stands for Mean Time Between Failures; it is the average time the system operates before a failure occurs.

The distinction is that MTTR relates to downtime (how long repairs take), while MTBF relates to uptime between failures (how long the system runs before something fails). A common way to connect them is through availability: Availability = MTBF / (MTBF + MTTR). For example, if a system runs for about 100 hours before a failure and takes about 5 hours to repair, availability is 100 / 105 ≈ 95.2%.

The phrasing that MTTR is a minimum time to restore or that MTBF is a maximum time before failure isn’t how these terms are defined. They use “Mean” (average) and “Between Failures” (the interval between successive failures) to convey typical behavior over many incidents.