Common cause and special cause (statistics) - Wikipedia

Shewhart and W. Edwards Deming. Briefly, "common causes", also called natural patterns, are the usual, historical, quantifiable variation in a system, while " ... Commoncauseandspecialcause(statistics) FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Thisarticleisaboutstatisticalpatterns.Forvisiblepatterns,seePatternsinnature. Thisarticleneedsadditionalcitationsforverification.Pleasehelpimprovethisarticlebyaddingcitationstoreliablesources.Unsourcedmaterialmaybechallengedandremoved.Findsources: "Commoncauseandspecialcause" statistics – news ·newspapers ·books ·scholar ·JSTOR(February2013)(Learnhowandwhentoremovethistemplatemessage) Typeofvariation Synonyms Commoncause ChancecauseNon-assignablecauseNoiseNaturalpattern Randomeffects Randomerror Specialcause AssignablecauseSignalUnnaturalpattern Systematiceffects Systematicerror Commonandspecialcausesarethetwodistinctoriginsofvariationinaprocess,asdefinedinthestatisticalthinkingandmethodsofWalterA.ShewhartandW.EdwardsDeming.Briefly,"commoncauses",alsocallednaturalpatterns,aretheusual,historical,quantifiablevariationinasystem,while"specialcauses"areunusual,notpreviouslyobserved,non-quantifiablevariation. Thedistinctionisfundamentalinphilosophyofstatisticsandphilosophyofprobability,withdifferenttreatmentoftheseissuesbeingaclassicissueofprobabilityinterpretations,beingrecognisedanddiscussedasearlyas1703byGottfriedLeibniz;variousalternativenameshavebeenusedovertheyears. ThedistinctionhasbeenparticularlyimportantinthethinkingofeconomistsFrankKnight,JohnMaynardKeynesandG.L.S.Shackle. Contents 1Originsandconcepts 2Definitions 2.1Common-causevariations 2.2Special-causevariation 3Examples 3.1Commoncauses 3.2Specialcauses 4Importancetoeconomics 5Importancetoindustrialandqualitymanagement 6Importancetostatistics 6.1DemingandShewhart 6.2Keynes 7Commonmodefailureinengineering 8Seealso 9Bibliography 10References Originsandconcepts[edit] In1703,JacobBernoulliwrotetoGottfriedLeibniztodiscusstheirsharedinterestinapplyingmathematicsandprobabilitytogamesofchance.Bernoullispeculatedwhetheritwouldbepossibletogathermortalitydatafromgravestonesandtherebycalculate,bytheirexistingpractice,theprobabilityofamancurrentlyaged20yearsoutlivingamanaged60years.Leibnizrepliedthathedoubtedthiswaspossible: Naturehasestablishedpatternsoriginatinginthereturnofeventsbutonlyforthemostpart.Newillnessesfloodthehumanrace,sothatnomatterhowmanyexperimentsyouhavedoneoncorpses,youhavenottherebyimposedalimitonthenatureofeventssothatinthefuturetheycouldnotvary. Thiscapturesthecentralideathatsomevariationispredictable,atleastapproximatelyinfrequency.Thiscommon-causevariationisevidentfromtheexperiencebase.However,new,unanticipated,emergentorpreviouslyneglectedphenomena(e.g."newdiseases")resultinvariationoutsidethehistoricalexperiencebase.ShewhartandDemingarguedthatsuchspecial-causevariationisfundamentallyunpredictableinfrequencyofoccurrenceorinseverity. JohnMaynardKeynesemphasisedtheimportanceofspecial-causevariationwhenhewrote: By"uncertain"knowledge...Idonotmeanmerelytodistinguishwhatisknownforcertainfromwhatisonlyprobable.Thegameofrouletteisnotsubject,inthissense,touncertainty...ThesenseinwhichIamusingthetermisthatinwhichtheprospectofaEuropeanwarisuncertain,orthepriceofcopperandtherateofinteresttwentyyearshence,ortheobsolescenceofanewinvention...Aboutthesemattersthereisnoscientificbasisonwhichtoformanycalculableprobabilitywhatever.Wesimplydonotknow! Definitions[edit] Common-causevariations[edit] Common-causevariationischaracterisedby:[citationneeded] Phenomenaconstantlyactivewithinthesystem; Variationpredictableprobabilistically; Irregularvariationwithinahistoricalexperiencebase;and Lackofsignificanceinindividualhighorlowvalues. Theoutcomesofaperfectlybalancedroulettewheelareagoodexampleofcommon-causevariation.Common-causevariationisthenoisewithinthesystem. WalterA.Shewhartoriginallyusedthetermchancecause.[1]ThetermcommoncausewascoinedbyHarryAlpertin1947.TheWesternElectricCompanyusedthetermnaturalpattern.[2]Shewhartcalledaprocessthatfeaturesonlycommon-causevariationasbeinginstatisticalcontrol.Thistermisdeprecatedbysomemodernstatisticianswhopreferthephrasestableandpredictable. Special-causevariation[edit] Special-causevariationischaracterisedby:[citationneeded] New,unanticipated,emergentorpreviouslyneglectedphenomenawithinthesystem; Variationinherentlyunpredictable,evenprobabilistically; Variationoutsidethehistoricalexperiencebase;and Evidenceofsomeinherentchangeinthesystemorourknowledgeofit. Special-causevariationalwaysarrivesasasurprise.Itisthesignalwithinasystem. WalterA.Shewhartoriginallyusedthetermassignablecause.[3]Thetermspecial-causewascoinedbyW.EdwardsDeming.TheWesternElectricCompanyusedthetermunnaturalpattern.[2] Examples[edit] Thissectiondoesnotciteanysources.Pleasehelpimprovethissectionbyaddingcitationstoreliablesources.Unsourcedmaterialmaybechallengedandremoved.(February2013)(Learnhowandwhentoremovethistemplatemessage) Commoncauses[edit] Inappropriateprocedures Poordesign Poormaintenanceofmachines Lackofclearlydefinedstandardoperatingprocedures Poorworkingconditions,e.g.lighting,noise,dirt,temperature,ventilation Substandardrawmaterials Measurementerror Qualitycontrolerror Vibrationinindustrialprocesses Ambienttemperatureandhumidity Normalwearandtear Variabilityinsettings Computerresponsetime Specialcauses[edit] Faultyadjustmentofequipment Operatorfallsasleep Defectivecontrollers Machinemalfunction Fallofground Computercrash Deficientbatchofrawmaterial Powersurges Highhealthcaredemandfromelderlypeople Brokenpart Insufficientawareness Abnormaltraffic(clickfraud)onwebads Extremelylonglabtestingturnovertimeduetoswitchingtoanewcomputersystem Operatorabsent[4] Importancetoeconomics[edit] Furtherinformation:Knightianuncertainty Ineconomics,thiscircleofideasisanalysedundertherubricof"Knightianuncertainty".JohnMaynardKeynesandFrankKnightbothdiscussedtheinherentunpredictabilityofeconomicsystemsintheirworkandusedittocriticisethemathematicalapproachtoeconomics,intermsofexpectedutility,developedbyLudwigvonMisesandothers.Keynesinparticulararguedthateconomicsystemsdidnotautomaticallytendtotheequilibriumoffullemploymentowingtotheiragents'inabilitytopredictthefuture.AsheremarkedinTheGeneralTheoryofEmployment,InterestandMoney: ...aslivingandmovingbeings,weareforcedtoact...[evenwhen]ourexistingknowledgedoesnotprovideasufficientbasisforacalculatedmathematicalexpectation. Keynes'thinkingwasatoddswiththeclassicalliberalismoftheAustrianSchoolofeconomists,butG.L.S.ShacklerecognisedtheimportanceofKeynes'sinsightandsoughttoformaliseitwithinafree-marketphilosophy. Infinancialeconomics,theblackswantheoryofNassimNicholasTalebisbasedonthesignificanceandunpredictabilityofspecialcauses. Importancetoindustrialandqualitymanagement[edit] Thissectiondoesnotciteanysources.Pleasehelpimprovethissectionbyaddingcitationstoreliablesources.Unsourcedmaterialmaybechallengedandremoved.(February2013)(Learnhowandwhentoremovethistemplatemessage) Aspecial-causefailureisafailurethatcanbecorrectedbychangingacomponentorprocess,whereasacommon-causefailureisequivalenttonoiseinthesystemandspecificactionscannotbemadetopreventthefailure. HarryAlpertobserved:[citationneeded] Ariotoccursinacertainprison.Officialsandsociologiststurnoutadetailedreportabouttheprison,withafullexplanationofwhyandhowithappenedhere,ignoringthefactthatthecauseswerecommontoamajorityofprisons,andthattheriotcouldhavehappenedanywhere. Thequoterecognisesthatthereisatemptationtoreacttoanextremeoutcomeandtoseeitassignificant,evenwhereitscausesarecommontomanysituationsandthedistinctivecircumstancessurroundingitsoccurrence,theresultsofmerechance.Suchbehaviourhasmanyimplicationswithinmanagement,oftenleadingtoadhocinterventionsthatmerelyincreasethelevelofvariationandfrequencyofundesirableoutcomes. DemingandShewhartbothadvocatedthecontrolchartasameansofmanagingabusinessprocessinaneconomicallyefficientmanner. Importancetostatistics[edit] DemingandShewhart[edit] Thissectiondoesnotciteanysources.Pleasehelpimprovethissectionbyaddingcitationstoreliablesources.Unsourcedmaterialmaybechallengedandremoved.(February2013)(Learnhowandwhentoremovethistemplatemessage) Withinthefrequencyprobabilityframework,thereisnoprocesswherebyaprobabilitycanbeattachedtothefutureoccurrenceofspecialcause.[citationneeded]OnemightnaivelyaskwhethertheBayesianapproachdoesallowsuchaprobabilitytobespecified.Theexistenceofspecial-causevariationledKeynesandDemingtoaninterestinBayesianprobability,butnoformalsynthesisemergedfromtheirwork.MoststatisticiansoftheShewhart-Demingschooltaketheviewthatspecialcausesarenotembeddedineitherexperienceorincurrentthinking(that'swhytheycomeasasurprise;theirpriorprobabilityhasbeenneglected—ineffect,assignedthevaluezero)sothatanysubjectiveprobabilityisdoomedtobehopelesslybadlycalibratedinpractice. ItisimmediatelyapparentfromtheLeibnizquoteabovethatthereareimplicationsforsampling.Demingobservedthatinanyforecastingactivity,thepopulationisthatoffutureeventswhilethesamplingframeis,inevitably,somesubsetofhistoricalevents.Demingheldthatthedisjointnatureofpopulationandsamplingframewasinherentlyproblematiconcetheexistenceofspecial-causevariationwasadmitted,rejectingthegeneraluseofprobabilityandconventionalstatisticsinsuch situations.Hearticulatedthedifficultyasthedistinctionbetweenanalyticandenumerativestatisticalstudies. Shewhartarguedthat,asprocessessubjecttospecial-causevariationwereinherentlyunpredictable,theusualtechniquesofprobabilitycouldnotbeusedtoseparatespecial-causefromcommon-causevariation.Hedevelopedthecontrolchartasastatisticalheuristictodistinguishthetwotypesofvariation.BothDemingandShewhartadvocatedthecontrolchartasameansofassessingaprocess'sstateofstatisticalcontrolandasafoundationforforecasting. Keynes[edit] Keynesidentifiedthreedomainsofprobability:[5] frequencyprobability; subjectiveorBayesianprobability;and eventslyingoutsidethepossibilityofanydescriptionintermsofprobability(specialcauses) andsoughttobaseaprobabilitytheorythereon. Commonmodefailureinengineering[edit] Thissectionpossiblycontainsoriginalresearch.Pleaseimproveitbyverifyingtheclaimsmadeandaddinginlinecitations.Statementsconsistingonlyoforiginalresearchshouldberemoved.(February2013)(Learnhowandwhentoremovethistemplatemessage) Commonmodefailurehasamorespecificmeaninginengineering.Itreferstoeventswhicharenotstatisticallyindependent.Failuresinmultiplepartsofasystemmaybecausedbyasinglefault,particularlyrandomfailuresduetoenvironmentalconditionsoraging.Anexampleiswhenallofthepumpsforafiresprinklersystemarelocatedinoneroom.Iftheroombecomestoohotforthepumpstooperate,theywillallfailatessentiallythesametime,fromonecause(theheatintheroom).[6]Anotherexampleisanelectronicsystemwhereinafaultinapowersupplyinjectsnoiseontoasupplyline,causingfailuresinmultiplesubsystems. Thisisparticularlyimportantinsafety-criticalsystemsusingmultipleredundantchannels.Iftheprobabilityoffailureinonesubsystemisp,thenitwouldbeexpectedthatanNchannelsystemwouldhaveaprobabilityoffailureofpN.However,inpractice,theprobabilityoffailureismuchhigherbecausetheyarenotstatisticallyindependent;forexampleionizingradiationorelectromagneticinterference(EMI)mayaffectallthechannels.[7] Theprincipleofredundancystatesthat,wheneventsoffailureofacomponentarestatisticallyindependent,theprobabilitiesoftheirjointoccurrencemultiply.[8]Thus,forinstance,iftheprobabilityoffailureofacomponentofasystemisoneinonethousandperyear,theprobabilityofthejointfailureoftwoofthemisoneinonemillionperyear,providedthatthetwoeventsarestatisticallyindependent.Thisprinciplefavorsthestrategyoftheredundancyofcomponents.OneplacethisstrategyisimplementedisinRAID1,wheretwoharddisksstoreacomputer'sdataredundantly. Butevensotherecanbemanycommonmodes:consideraRAID1wheretwodisksarepurchasedonlineandareinstalledinacomputer,therecanbemanycommonmodes: Thedisksarelikelytobefromthesamemanufacturerandofthesamemodel,thereforetheysharethesamedesignflaws. Thedisksarelikelytohavesimilarserialnumbers,thustheymayshareanymanufacturingflawsaffectingproductionofthesamebatch. Thedisksarelikelytohavebeenshippedatthesametime,thustheyarelikelytohavesufferedfromthesametransportationdamage. Asinstalledbothdisksareattachedtothesamepowersupply,makingthemvulnerabletothesamepowersupplyissues. Asinstalledbothdisksareinthesamecase,makingthemvulnerabletothesameoverheatingevents. Theywillbebothattachedtothesamecardormotherboard,anddrivenbythesamesoftware,whichmayhavethesamebugs. BecauseoftheverynatureofRAID1,bothdiskswillbesubjectedtothesameworkloadandverycloselysimilaraccesspatterns,stressingtheminthesameway. Also,iftheeventsoffailureoftwocomponentsaremaximallystatisticallydependent,theprobabilityofthejointfailureofbothisidenticaltotheprobabilityoffailureofthemindividually.Insuchacase,theadvantagesofredundancyarenegated.Strategiesfortheavoidanceofcommonmodefailuresincludekeepingredundantcomponentsphysicallyisolated. Aprimeexampleofredundancywithisolationisanuclearpowerplant.[9][10]ThenewABWRhasthreedivisionsofEmergencyCoreCoolingSystems,eachwithitsowngeneratorsandpumpsandeachisolatedfromtheothers.ThenewEuropeanPressurizedReactorhastwocontainmentbuildings,oneinsidetheother.However,evenhereitispossibleforacommonmodefailuretooccur(forexample,intheFukushimaDaiichiNuclearPowerPlant,mainspowerwasseveredbytheTōhokuearthquake,thenthethirteenbackupdieselgeneratorswereallsimultaneouslydisabledbythesubsequenttsunamithatfloodedthebasementsoftheturbinehalls). Seealso[edit] Correctiveandpreventiveaction(CAPA) Nuclearsafety Probabilisticriskassessment Statisticalprocesscontrol Bibliography[edit] Deming,W.E.(1975)Onprobabilityasabasisforaction,TheAmericanStatistician,29(4),pp. 146–152 Deming,W.E.(1982)OutoftheCrisis:Quality,ProductivityandCompetitivePositionISBN 0-521-30553-5 Keynes,J.M.(1936)TheGeneralTheoryofEmployment,InterestandMoneyISBN 1-57392-139-4 Keynes,J.M.(1921)[5] Knight,F.H.(1921)Risk,UncertaintyandProfitISBN 1-58798-126-2 Shackle,G.L.S.(1972)EpistemicsandEconomics:ACritiqueofEconomicDoctrinesISBN 1-56000-558-0 Shewhart,W.A.(1931)EconomicControlofQualityofManufacturedProductISBN 0-87389-076-0 Shewhart,W.A.(1939)StatisticalMethodfromtheViewpointofQualityControlISBN 0-486-65232-7 Wheeler,D.J.&Chambers,D.S.(1992)UnderstandingStatisticalProcessControlISBN 0-945320-13-2 References[edit] ^Shewhart,WalterA.(1931)."Economiccontrolofqualityofmanufacturedproduct".NewYorkCity:D.VanNostrandCompany,Inc:7.OCLC 1045408.Citejournalrequires|journal=(help) ^abWesternElectricCompany(1956)."IntroductiontoStatisticalQualityControlhandbook"(1 ed.).Indianapolis,Indiana:WesternElectricCo:23–24.OCLC 33858387.Citejournalrequires|journal=(help) ^Shewhart,WalterA.(1931)."Economiccontrolofqualityofmanufacturedproduct".NewYorkCity:D.VanNostrandCompany,Inc:14.OCLC 1045408.Citejournalrequires|journal=(help) ^"StatisticalInference".Archivedfromtheoriginalon7October2006.Retrieved13November2006. ^abKeynes,J.M.(1921).ATreatiseonProbability.ISBN 0-333-10733-0. ^Thomson,Jim(February2012)."Common-ModeFailureConsiderationsinHigh-IntegrityC&ISystems"(PDF).SafetyinEngineering.Retrieved21November2012. ^Randell,BrianDesignFaultToleranceinTheEvolutionofFault-TolerantComputing,Avizienis,A.;Kopetz,H.;Laprie,J.-C.(eds.),pp.251–270.Springer-Verlag,1987.ISBN 3-211-81941-X. ^"SEIFramework:FaultToleranceMechanisms".RedundancyManagement.NISTHighIntegritySoftwareSystemsAssurance.30March1995.Archivedfromtheoriginalon24November2012.Retrieved21November2012. ^Edwards,G.T.;Watson,I.A.(July1979)."AStudyofCommon-ModeFailures".SRDR146.UKAtomicEnergyAuthority:SafetyandReliabilityDirectorate. ^Bourne,A.J.;Edwards,G.T.;Hunns,D.M.;Poulter,D.R.;Watson,I.A.(January1981)."DefencesagainstCommon-ModeFailuresinRedundancySystems–AGuideforManagement,DesignersandOperators".SRDR196.UKAtomicEnergyAuthority:SafetyandReliabilityDirectorate. 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