902:. Failures in multiple parts of a system may be caused by a single fault, particularly random failures due to environmental conditions or aging. An example is when all of the pumps for a fire sprinkler system are located in one room. If the room becomes too hot for the pumps to operate, they will all fail at essentially the same time, from one cause (the heat in the room). Another example is an electronic system wherein a fault in a power supply injects noise onto a supply line, causing failures in multiple subsystems.
43:
839:
734:, but no formal synthesis emerged from their work. Most statisticians of the Shewhart-Deming school take the view that special causes are not embedded in either experience or in current thinking (that's why they come as a surprise; their prior probability has been neglected—in effect, assigned the value zero) so that any subjective probability is doomed to be hopelessly badly
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states that, when events of failure of a component are statistically independent, the probabilities of their joint occurrence multiply. Thus, for instance, if the probability of failure of a component of a system is one in one thousand per year, the probability of the joint failure of two of them is
761:
of historical events. Deming held that the disjoint nature of population and sampling frame was inherently problematic once the existence of special-cause variation was admitted, rejecting the general use of probability and conventional statistics in such situations. He articulated the difficulty as
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Also, if the events of failure of two components are maximally statistically dependent, the probability of the joint failure of both is identical to the probability of failure of them individually. In such a case, the advantages of redundancy are negated. Strategies for the avoidance of common mode
315:
By "uncertain" knowledge ... I do not mean merely to distinguish what is known for certain from what is only probable. The game of roulette is not subject, in this sense, to uncertainty ... The sense in which I am using the term is that in which the prospect of a
European war is uncertain, or the
282:
Nature has established patterns originating in the return of events but only for the most part. New illnesses flood the human race, so that no matter how many experiments you have done on corpses, you have not thereby imposed a limit on the nature of events so that in the future they could not
678:
Alpert recognises that there is a temptation to react to an extreme outcome and to see it as significant, even where its causes are common to many situations and the distinctive circumstances surrounding its occurrence, the results of mere chance. Such behaviour has many implications within
672:
A riot occurs in a certain prison. Officials and sociologists turn out a detailed report about the prison, with a full explanation of why and how it happened here, ignoring the fact that the causes were common to a majority of prisons, and that the riot could have happened
316:
price of copper and the rate of interest twenty years hence, or the obsolescence of a new invention ... About these matters there is no scientific basis on which to form any calculable probability whatever. We simply do not know!
660:
A special-cause failure is a failure that can be corrected by changing a component or process, whereas a common-cause failure is equivalent to noise in the system and specific actions cannot be made to prevent the failure.
772:
argued that, as processes subject to special-cause variation were inherently unpredictable, the usual techniques of probability could not be used to separate special-cause from common-cause variation. He developed the
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But even so, a system can have many common modes of failure. For example, consider the common modes of failure of a RAID1 where two disks are purchased from an online store and installed in a computer:
937:
one in one million per year, provided that the two events are statistically independent. This principle favors the strategy of the redundancy of components. One place this strategy is implemented is in
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is evident from the experience base. However, new, unanticipated, emergent or previously neglected phenomena (e.g. "new diseases") result in variation outside the historical experience base.
60:
967:
Because of the very nature of RAID1, both disks will be subjected to the same workload and very closely similar access patterns, stressing them in the same way.
620:... as living and moving beings, we are forced to act ... our existing knowledge does not provide a sufficient basis for a calculated mathematical expectation.
597:
both discussed the inherent unpredictability of economic systems in their work and used it to criticise the mathematical approach to economics, in terms of
611:
1003:, then the thirteen backup diesel generators were all simultaneously disabled by the subsequent tsunami that flooded the basements of the turbine halls).
210:, are the usual, historical, quantifiable variation in a system, while "special causes" are unusual, not previously observed, non-quantifiable variation.
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data from gravestones and thereby calculate, by their existing practice, the probability of a man currently aged 20 years outliving a man aged 60 years.
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79:
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229:; various alternative names have been used over the years. The distinction has been particularly important in the thinking of economists
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The disks are likely to have similar serial numbers, thus they may share any manufacturing flaws affecting production of the same batch.
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The disks are likely to have been shipped at the same time, thus they are likely to have suffered from the same transportation damage.
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management, often leading to ad hoc interventions that merely increase the level of variation and frequency of undesirable outcomes.
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However, in practice, the probability of failure is much higher because they are not statistically independent; for example
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They will be both attached to the same card or motherboard, and driven by the same software, which may have the same bugs.
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The disks are likely to be from the same manufacturer and of the same model, therefore they share the same design flaws.
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As installed both disks are attached to the same power supply, making them vulnerable to the same power supply issues.
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and others. Keynes in particular argued that economic systems did not automatically tend to the equilibrium of
64:
1405:
Defences against Common-Mode
Failures in Redundancy Systems – A Guide for Management, Designers and Operators
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This captures the central idea that some variation is predictable, at least approximately in frequency. This
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approach does allow such a probability to be specified. The existence of special-cause variation led
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As installed both disks are in the same case, making them vulnerable to the same overheating events.
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events lying outside the possibility of any description in terms of probability (special causes)
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can be attached to the future occurrence of special cause. One might naively ask whether the
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Bourne, A. J.; Edwards, G. T.; Hunns, D. M.; Poulter, D. R.; Watson, I. A. (January 1981).
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1340:, Avizienis, A.; Kopetz, H.; Laprie, J.-C. (eds.), pp. 251–270. Springer-Verlag, 1987.
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recognised the importance of Keynes's insight and sought to formalise it within a
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Extremely long lab testing turnover time due to switching to a new computer system
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wheel are a good example of common-cause variation. Common-cause variation is the
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New, unanticipated, emergent or previously neglected phenomena within the system;
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1365:. NIST High Integrity Software Systems Assurance. 30 March 1995. Archived from
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386:. Shewhart called a process that features only common-cause variation as being
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has a more specific meaning in engineering. It refers to events which are not
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Poor working conditions, e.g. lighting, noise, dirt, temperature, ventilation
393:. This term is deprecated by some modern statisticians who prefer the phrase
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1407:. UK Atomic Energy Authority: Safety and Reliability Directorate. SRD R196.
1392:. UK Atomic Energy Authority: Safety and Reliability Directorate. SRD R146.
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owing to their agents' inability to predict the future. As he remarked in
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is fundamentally unpredictable in frequency of occurrence or in severity.
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are the two distinct origins of variation in a process, as defined in the
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This is particularly important in safety-critical systems using multiple
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advocated the control chart as a means of assessing a process's state of
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1307:"Common-Mode Failure Considerations in High-Integrity C&I Systems"
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Evidence of some inherent change in the system or our knowledge of it.
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This article is about statistical patterns. For visible patterns, see
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is based on the significance and unpredictability of special causes.
867:. Statements consisting only of original research should be removed.
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In economics, this circle of ideas is analysed under the rubric of "
221:, with different treatment of these issues being a classic issue of
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failures include keeping redundant components physically isolated.
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emphasised the importance of special-cause variation when he wrote:
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Out of the Crisis: Quality, Productivity and
Competitive Position
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Special-cause variation always arrives as a surprise. It is the
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941:, where two hard disks store a computer's data redundantly.
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channels. If the probability of failure in one subsystem is
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Irregular variation within a historical experience base; and
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1230:. New York City: D. Van Nostrand Company, Inc. p. 14.
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Deming, W. E. (1975) On probability as a basis for action,
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to distinguish the two types of variation. Both Deming and
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Variation inherently unpredictable, even probabilistically;
1167:. New York City: D. Van Nostrand Company, Inc. p. 7.
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Epistemics and
Economics: A Critique of Economic Doctrines
1188:
1122:
Statistical Method from the
Viewpoint of Quality Control
749:. Deming observed that in any forecasting activity, the
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channel system would have a probability of failure of
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Lack of significance in individual high or low values.
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Variation outside the historical experience base; and
225:, being recognised and discussed as early as 1703 by
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The
General Theory of Employment, Interest and Money
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The
General Theory of Employment, Interest and Money
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Economic control of quality of manufactured product
1165:
Economic control of quality of manufactured product
1109:
Economic
Control of Quality of Manufactured Product
67:. Unsourced material may be challenged and removed.
975:A prime example of redundancy with isolation is a
269:speculated whether it would be possible to gather
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1204:, Indiana: Western Electric Co. pp. 23–24.
76:"Common cause and special cause" statistics
656:Importance to industrial and quality management
335:Phenomena constantly active within the system;
441:. The Western Electric Company used the term
405:Special-cause variation is characterised by:
257:to discuss their shared interest in applying
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1133:Wheeler, D. J. & Chambers, D. S. (1992)
764:analytic and enumerative statistical studies
745:quote above that there are implications for
429:Walter A. Shewhart originally used the term
331:Common-cause variation is characterised by:
1388:Edwards, G. T.; Watson, I. A. (July 1979).
1359:"SEI Framework: Fault Tolerance Mechanisms"
700:
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551:High healthcare demand from elderly people
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277:replied that he doubted this was possible:
1338:The Evolution of Fault-Tolerant Computing
1268:
1135:Understanding Statistical Process Control
883:Learn how and when to remove this message
800:identified three domains of probability:
714:framework, there is no process whereby a
127:Learn how and when to remove this message
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206:. Briefly, "common causes", also called
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625:Keynes' thinking was at odds with the
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789:and as a foundation for forecasting.
697:in an economically efficient manner.
352:The outcomes of a perfectly balanced
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913:, then it would be expected that an
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741:It is immediately apparent from the
65:adding citations to reliable sources
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929:(EMI) may affect all the channels.
753:is that of future events while the
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829:Common mode failure in engineering
213:The distinction is fundamental in
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999:, mains power was severed by the
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492:Vibration in industrial processes
1013:Corrective and preventive action
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495:Ambient temperature and humidity
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1390:A Study of Common-Mode Failures
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543:Deficient batch of raw material
52:needs additional citations for
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1305:Thomson, Jim (February 2012).
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985:Emergency Core Cooling Systems
520:Faulty adjustment of equipment
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1:
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1024:Probabilistic risk assessment
470:standard operating procedures
1083:Risk, Uncertainty and Profit
989:European Pressurized Reactor
927:electromagnetic interference
465:Poor maintenance of machines
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1429:Statistical process control
1424:Probability interpretations
1196:Statistical Quality Control
1029:Statistical process control
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863:the claims made and adding
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223:probability interpretations
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1034:There are unknown unknowns
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1281:A Treatise on Probability
1094:Shackle, G. L. S. (1972)
1049:, 29(4), pp. 146–152
1047:The American Statistician
900:statistically independent
792:
693:as a means of managing a
477:Substandard raw materials
366:originally used the term
219:philosophy of probability
192:Common and special causes
1434:Philosophy of statistics
1190:Western Electric Company
762:the distinction between
701:Importance to statistics
468:Lack of clearly defined
459:Inappropriate procedures
380:Western Electric Company
215:philosophy of statistics
1312:. Safety in Engineering
1250:"Statistical Inference"
1120:Shewhart, W. A. (1939)
1107:Shewhart, W. A. (1931)
983:has three divisions of
934:principle of redundancy
575:Importance to economics
504:Variability in settings
401:Special-cause variation
327:Common-cause variations
302:special-cause variation
1334:Design Fault Tolerance
1278:Keynes, J. M. (1921).
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557:Insufficient awareness
395:stable and predictable
338:Variation predictable
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290:common-cause variation
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1363:Redundancy Management
1081:Knight, F. H. (1921)
1065:Keynes, J. M. (1936)
1052:Deming, W. E. (1982)
993:containment buildings
821:and sought to base a
757:is, inevitably, some
712:frequency probability
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587:Knightian uncertainty
581:Knightian uncertainty
579:Further information:
529:Defective controllers
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1078:Keynes, J. M. (1921)
812:Bayesian probability
732:Bayesian probability
627:classical liberalism
265:to games of chance.
245:Origins and concepts
196:statistical thinking
157:Non-assignable cause
61:improve this article
1444:Applied mathematics
1369:on 24 November 2012
1224:Shewhart, Walter A.
1161:Shewhart, Walter A.
977:nuclear power plant
896:Common mode failure
787:statistical control
706:Deming and Shewhart
689:both advocated the
646:financial economics
633:of economists, but
591:John Maynard Keynes
532:Machine malfunction
390:statistical control
360:within the system.
308:John Maynard Keynes
235:John Maynard Keynes
181:Systematic effects
18:Common mode failure
923:ionizing radiation
848:possibly contains
823:probability theory
730:to an interest in
560:Abnormal traffic (
364:Walter A. Shewhart
200:Walter A. Shewhart
179:Unnatural pattern
32:Patterns in nature
27:Statistics concept
1256:on 7 October 2006
1001:TĹŤhoku earthquake
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850:original research
777:as a statistical
650:black swan theory
443:unnatural pattern
439:W. Edwards Deming
426:within a system.
340:probabilistically
300:argued that such
255:Gottfried Leibniz
227:Gottfried Leibniz
204:W. Edwards Deming
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184:Systematic error
144:Type of variation
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547:Power surges
525:falls asleep
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59:Please help
54:verification
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1373:21 November
1316:21 November
1260:13 November
716:probability
710:Within the
639:free-market
562:click fraud
554:Broken part
481:Measurement
462:Poor design
433:. The term
370:. The term
322:Definitions
263:probability
259:mathematics
1418:Categories
1148:References
979:. The new
857:improve it
751:population
736:calibrated
87:newspapers
907:redundant
861:verifying
825:thereon.
805:frequency
779:heuristic
673:anywhere.
667:observed:
523:Operator
507:Computer
271:mortality
267:Bernoulli
253:wrote to
249:In 1703,
147:Synonyms
1226:(1931).
1210:33858387
1198:handbook
1192:(1956).
1163:(1931).
1007:See also
991:has two
783:Shewhart
770:Shewhart
747:sampling
720:Bayesian
687:Shewhart
449:Examples
354:roulette
294:Shewhart
1236:1045408
1173:1045408
855:Please
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629:of the
498:Normal
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939:RAID 1
798:Keynes
793:Keynes
759:subset
728:Deming
724:Keynes
683:Deming
648:, the
424:signal
298:Deming
177:Signal
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1310:(PDF)
814:; and
489:error
483:error
358:noise
283:vary.
159:Noise
108:JSTOR
94:books
1375:2012
1342:ISBN
1318:2012
1286:ISBN
1262:2006
1232:OCLC
1206:OCLC
1169:OCLC
1138:ISBN
1125:ISBN
1112:ISBN
1099:ISBN
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1057:ISBN
981:ABWR
932:The
726:and
685:and
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1336:in
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