Mean time between failures (MTBF) is the predicted elapsed time between inherent failures of a mechanical or electronic system during normal system operation1. MTBF can be calculated as the arithmetic mean (average) time between failures of a system1. MTBF is a useful measure of reliability, because it indicates how long a system is likely to work before failing. The higher the MTBF, the more reliable the system2. Reliability is the probability that a system will perform its intended function without failure for a specified period of time under specified conditions3.

The other statements about MTBF are false. MTBF is not used for non-repairable products, but for repairable systems. For non-repairable products, mean time to failure (MTTF) is used instead4. MTTF is the expected time to failure for a non-repairable system1. An increase in MTBF is not proportional to an increase in quality, because quality is not only determined by reliability, but also by other factors such as performance, functionality, durability, and customer satisfaction5. MTBF is not the same as operating life or service life, because operating life or service life is the total time that a system can operate before it reaches the end of its useful life, while MTBF is the average time between failures during the operating life6.