Mean time between failures (MTBF) is the average time that elapses from one unplanned breakdown to the next, under normal operating conditions. It is a useful measure of reliability because it indicates how long a repairable system typically operates before failing. Reliability is the absence of unplanned downtime, and MTBF measures how often a system stops performing as expected. The other statements are not true about MTBF. MTBF is not used for non-repairable products, as they cannot be fixed and put back into operation after a failure. For non-repairable products, mean time to failure (MTTF) is used as a measure of reliability. MTBF is not proportional to quality, as quality is a broader concept that encompasses not only reliability, but also performance, durability, and customer satisfaction. MTBF is not the same as operating life or service life, as they refer to the total time that a system can function before reaching the end of its useful life, while MTBF refers to the average time between failures within the operating life. References: Mean Time Between Failures (MTBF): How to Calculate & Increase, APICS CPIM 8 Planning and Inventory Management | ASCM