The Mean Life Function

The mean life function, which provides a measure of the average time of operation to failure, is given by:

(5)

It should be noted that this is the expected or average time-to-failure and is denoted as the MTBF (Mean-Time-Before Failure) and is also called MTTF (Mean-Time-To-Failure) by many authors. (Note: The term MTBF has also been widely translated as Mean-Time-Between-Failure. For the mean life function, the only time that the Mean-Time-Between-Failure translation is correct is when the underlying distribution has a constant failure rate (e.g. exponential). In this case, the Mean-Time-To-Failure and Mean-Time-Between-Failure is identical; however, if the failure rate is not constant, then these terms are not equal.)