Which statement correctly differentiates MTBF and MTTF and their usage?

• A. MTBF is the mean time between failures for repairable systems; MTTF is the mean time to failure for non-repairable items; use MTBF when the item can be repaired, MTTF when the item is discarded after failure.
• B. MTBF equals the time to failure; MTTF equals time between failures.
• C. MTBF and MTTF are interchangeable and used identically.
• D. MTBF is the total time to failure; MTTF is the time to repair.

Answer: (A)

MTBF and MTTF hinge on whether the item is repairable after a failure. MTBF is the average time between failures for a system that is repaired and put back into service after each failure; it focuses on the interval between one failure and the next in a repairable setup. MTTF is the average time to failure for items that, once they fail, are not repaired (they’re replaced or discarded); it measures how long such a non-repairable item lasts before failing.

Because of this, they aren’t interchangeable. MTBF informs maintenance planning, spare parts stocking, and reliability modeling for repairable equipment, while MTTF provides life expectancy for non-repairable components. The other statements mix up the concepts—confusing time to failure with time between failures, treating MTBF/MTTF as interchangeable, or swapping “time to failure” with “time to repair.”