Which of the following statements correctly defines MTTR and MTBF and notes their role in maintenance planning?

• A. MTTR is time to respond to incidents; MTBF is mean time between faults; They determine energy usage.
• B. MTTR is mean time to repair (downtime during restoration); MTBF is mean time between failures (average uptime between failures). Together they inform availability targets, maintenance windows, redundancy needs, and reliability improvements.
• C. MTTR refers to data backup; MTBF to data integrity; They set backup frequency.
• D. MTTR is mean time to reset; MTBF is mean time between failures; They determine capacity planning.

Answer: (B)

MTTR and MTBF are two complementary metrics used to plan maintenance and measure system reliability. MTBF, or mean time between failures, represents the average time the system operates before a failure occurs, i.e., the uptime between failures. MTTR, or mean time to repair, is the average time required to repair a failed component and restore service, i.e., the downtime during restoration. Together they inform availability targets and maintenance decisions: availability is commonly approximated as MTBF / (MTBF + MTTR). A shorter MTTR means faster recovery and higher availability, while a longer MTBF means fewer interruptions. These metrics help determine appropriate maintenance windows, redundancy needs, spare parts inventory, and where reliability improvements should focus—whether on reducing the failure rate (increasing MTBF) or speeding repairs (reducing MTTR). Misdefinitions like MTTR as incident response time or MTBF as data backup frequency don’t fit because MTTR is about repair time after a failure and MTBF is about time between failures, not backup cadence or data integrity.