Mean time between failures (MTBF)'s the arithmetic mean (average) time between failures of a system. The MTBF's typically part of a model that assumes the failed system's immediately repaired (zero elapsed time), as a part of a renewal process. This's in contrast to the mean time to failure (MTTF), which measures average time between failure with the modeling assumption that the failed system isn't repaired. Overview For each observation, downtime's the instantaneous time it went down, which'… (More on Mtbf)