Differentiate MTBF and MTTR with respect to system reliability.

• A. MTBF is the time to repair after a failure.
• B. MTTR measures the time to prevent failures from occurring.
• C. MTBF is the frequency of maintenance events.
• D. MTBF is the expected time between failures for a system; MTTR is the time to recover after a failure.

Answer: (D)

Understanding reliability and maintainability together helps explain these two metrics. MTBF is the expected time between failures—the typical interval you can expect before something in the system fails again. A longer MTBF means failures occur less often, signaling higher reliability during normal operation.

MTTR, on the other hand, is the time to recover after a failure—the duration required to repair and restore functionality. A shorter MTTR means the system spends less time down after a failure, signaling better maintainability and faster recovery.

These two together also influence overall availability, often approximated by MTBF divided by MTBF plus MTTR. So, a system with a long time between failures and a quick repair time will be up more of the time.

In short, MTBF is the interval between failures; MTTR is the time to recover after a failure.