Editing Failure rate (section)

== Mean time between failures (MTBF) ==

The [[mean time between failures]] (MTBF, <math>1/\lambda</math>) is often reported instead of the failure rate, as numbers such as "2,000 hours" are more intuitive than numbers such as "0.0005 per hour".

However, this is only valid if the failure rate <math>\lambda(t)</math> is actually constant over time, such as within the flat region of the bathtub curve. In many cases where MTBF is quoted, it refers only to this region; thus it cannot be used to give an accurate calculation of the average lifetime of a system, as it ignores the "burn-in" and "wear-out" regions.

MTBF appears frequently in [[engineering]] design requirements, and governs the frequency of required system maintenance and inspections. A similar ratio used in the [[transport industry|transport industries]], especially in [[railway]]s and trucking, is "mean distance between failures" - allowing maintenance to be scheduled based on distance travelled, rather than at regular time intervals.