Zaznaczyłem, że chodzi o teorię niezawodności:
(...) The formula for calculating the MTBF (Mean Time Between Failures) is MTBF= T/R where T = total time and R = number of failures
MTTF stands for Mean Time To Failure. To distinguish between the two, the concept of suspensions must first be understood. In reliability calculations, a suspension occurs when a destructive test or observation has been completed without observing a failure. MTBF calculations do not consider suspensions whereas MTTF does. MTTF is the number of total hours of service of all devices divided by the number of devices. It is only when all the parts fail with the same failure mode that MTBF converges to MTTF (...).
Nie jestem pewien, czy ktoś nie przetłumaczył po swojemu skrótów MTBF i MTTF. Jako taki "czas awarii" można przełożyć na mnóstwo sposobów.
https://en.wikipedia.org/wiki/Mean_time_between_failures