Everything about Mtbf totally explained
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 time to repair (MTTR).
Overview
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The MTTR can be similarly derived from the repair rate.
A common misconception about the MTBF is that it specifies the time (on average) when the probability of failure equals the probability of not having a failure. This is only true for certain symmetric distributions. In many cases, such as the (non-symmetric)
exponential distribution, this isn't the case. In particular, for an exponential failure distribution, the probability that an item will fail after an MTBF is approximately 0.63. For typical distributions with some variance, MTBF only represents a top-level aggregate statistic, and thus isn't suitable for predicting specific time to failure, the uncertainty arising from the variability in the time-to-failure distribution.
On commercial product descriptions, the "
MTTF lifetime" is the amount of time the product should last, assuming that it's used properly.
Variations of MTBF
There are many variations of MTBF, such as
mean time between system aborts (MTBSA) or
mean time between critical failures (MTBCF) or
mean time between unit replacement (MTBUR). Such nomenclature is used when it's desirable to differentiate among types of failures, such as critical and non-critical failures. For example, in an automobile, the failure of the FM radio doesn't prevent the primary operation of vehicle.
Mean time to failure (MTTF) is sometimes used instead of MTBF in cases where a system is replaced after a failure, since MTBF denotes time between failures in a system which is repaired.
Problems with MTBF
As of 1995, the use of MTBF in the
aeronautical industry (and others) has been called into question due to the inaccuracy of its application to real systems and the nature of the culture which it engenders. Many component MTBFs are given in databases, and often these values are very inaccurate.
This has led to the negative exponential distribution being used much more than it should have been. Some estimates say that only 40% of components have failure rates described by this. It has also been corrupted into the notion of an "acceptable" level of failures, which removes the desire to get to the root cause of a problem and take measures to delete it. The British
Royal Air Force is looking at other methods to describe reliability, such as
maintenance-free operating period (MFOP).
Further Information
Get more info on 'Mtbf'.
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