Mean time between failures (MTBF) is the mean (average) time between failures of a system, and is often attributed to the "useful life" of the device for example not including 'infant mortality' or 'end of life' if the device isn't repairable. Calculations of MTBF assume that a system is "renewed", for example fixed, after each failure, and then returned to service immediately after failure. The average time between failing and being returned to service is termed mean down time (MDT) or mean ti… (More on Mtbf)