Is Mean Time to Failure (MTTF) Equal to Mean Time of Life for Unrepairable Systems?

Abstract

One of the most important reliability parameters is the mean time to failure (MTTF). It is widely accepted that the MTTF is equal to the mean time of life E T . This article shows that this is not necessarily true. Although for the most commonly used statistical distributions (such as exponential, Gaussian, chi-square, Fisher-Tippett distributions) the values of MTTF and E T are equal, this is not the case for the log-normal distribution. Similarity, some less commonly used distributions (such as Breit-Wigner distribution) may also require calculation adjustments resulting from MTTF ≠ E T . Ignoring this discrepancy, an erroneous MTTF value can be obtained.