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| '''Mean time between failures (MTBF)''' is the predicted elapsed time between inherent failures of a system during operation.<ref>Jones, James V., ''Integrated Logistics Support Handbook'', page 4.2</ref> MTBF can be calculated as the [[arithmetic mean]] (average) time between [[failure]]s of a system. The MTBF is typically part of a model that assumes the failed system is immediately repaired ([[mean time to repair]], or MTTR), as a part of a [[renewal process]]. This is in contrast to the mean time to failure (MTTF), which measures average time to [[failure]]s with the modeling assumption that the failed system is not repaired (infinite repair time).
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| The definition of MTBF depends on the definition of what is considered a system [[failure]]. For complex, [[repairable]] systems, failures are considered to be those out of design conditions which place the system out of service and into a state for repair. Failures which occur that can be left or maintained in an unrepaired condition, and do not place the system out of service, are not considered failures under this definition.<ref>Colombo, A.G., and Sáiz de Bustamante, Amalio: ''Systems reliability assessment – Proceedings of the Ispra Course held at the Escuela Tecnica Superior de Ingenieros Navales, Madrid, Spain, September 19–23, 1988 in collaboration with Universidad Politecnica de Madrid'', 1988</ref> In addition, units that are taken down for routine scheduled maintenance or inventory control are not considered within the definition of failure.
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| == The Overview ==
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| [[Image:Time between failures.svg]]
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| For each observation, the "down time" is the instantaneous time it went down, which is after (i.e. greater than) the moment it went up, the "up time". The difference ("down time" minus "up time") is the amount of time it was operating between these two events.
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| MTBF value prediction is an important element in the development of products.
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| Reliability engineers and design engineers often use reliability software to calculate a product's MTBF according to various methods and standards (MIL-HDBK-217F, Telcordia SR332, Siemens Norm, FIDES,UTE 80-810 (RDF2000), etc.). However, these "prediction" methods are not intended to reflect fielded MTBF as is commonly believed; the intent of these tools is to focus design efforts on the weak links in the design.
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| == Formal definition of MTBF ==
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| By referring to the figure above, the MTBF is the sum of the operational periods divided by the number of observed failures. If the "Down time" (with space) refers to the start of "downtime" (without space) and "up time" (with space) refers to the start of "uptime" (without space), the formula will be:
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| :<math>\text{Mean time between failures}
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| = \text{MTBF} =\frac{\sum{(\text{start of downtime} - \text{start of uptime})}}\text{number of failures}. \!</math>
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| The MTBF is often denoted by the [[Theta|Greek letter ''θ'']], or
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| :<math>\text{MTBF} = \theta. \!</math>
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| The MTBF can be defined in terms of the [[expected value]] of the [[density function]] ''ƒ''(''t'')
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| :<math>\text{MTBF} = \int_0^\infty tf(t)\, dt \!</math>
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| where ''ƒ'' is the density function of time until failure – satisfying the standard requirement of density functions –
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| :<math>\int_0^\infty f(t)\, dt=1. \!</math>
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| In this context (of [[Reliability (engineering)|reliability]]) is density function ''ƒ''(''t'') also often referred as [[reliability function]] ''R''(''t'').
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| == Variations of MTBF ==
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| 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 is 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 does not prevent the primary operation of the 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. MTTFd is an extension of MTTF, where MTTFd is only concerned about failures which would result in a dangerous condition.
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| === MTTF and MTTFd calculation ===
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| :<math> \text{MTTF} \approx \frac{B_{10}}{0.1n_{op}},</math> | |
| :<math> \text{MTTFd} \approx \frac{B_{10d}}{0.1n_{op}},</math> | |
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| where B<sub>10</sub> is the number of operations that a device will operate prior to 10% of a sample of those devices would fail. B<sub>10d</sub> is the same calculation, but where 10% of the sample would fail to danger. n<sub>op</sub> is the number of operations/cycles.<ref>{{cite web|title=B10d Assessment - Reliability Parameter for Electro-Mechanical Components|url=http://www.tuv.com/web/media_get.php?mediaid=29575&fileid...2|publisher=TUVRheinland|accessdate=16 April 2012}}{{dead link|date=March 2013}}</ref>
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| ==Notes==
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| {{Reflist}}
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| == See also ==
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| *[[Failure rate]]
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| *[[Annualized failure rate]]
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| *[[Mean time to repair]]
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| *[[Power-On Hours]]
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| ==References==
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| * Jones, James V., ''Integrated Logistics Support Handbook'', McGraw–Hill Professional, 3rd edition (June 8, 2006), ISBN 0-07-147168-5
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| ==External links==
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| *[http://www.repairfaq.org/REPAIR/F_monfaqf.html#MONFAQF_002 MTBF and Life Expectancy on Monitor]
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| *[http://www.crouzet-usa.com/techtalk/lofiversion/index.php?t114.html MTBF and Life Expectancy on I/O Modules]
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| *[http://dearauthor.com/ebooks/its-april-fools-day-have-you-backed-up-your-ebooks/ MTBF and Life Expectancy on Harddisk]
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| *[http://www.irrigationcraft.com/b.htm MTBF and Life Expectancy on Mechanical Craft]
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| *[http://www.eventhelix.com/RealtimeMantra/FaultHandling/reliability_availability_basics.htm Reliability and Availability Basics]
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| *[http://www.vicr.com/documents/quality/Rel_MTBF.pdf Summary including MTTF discussion]
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| *[http://www.industrial-ebooks.com/spread_sheet/MTBCF_Calculation_Example250.php MTBCF Example spreadsheet]
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| *[http://www.mathpages.com/home/kmath498/kmath498.htm "Failure Rates, MTBF, and All That" at MathPages]
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| {{DEFAULTSORT:Mean Time Between Failures}}
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| [[Category:Failure]]
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| [[Category:Survival analysis]]
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| [[Category:Reliability analysis]]
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