Assignable Cause Definition - iSixSigma

Assignable cause, also known as a special cause, is one of the two types of variation a control chart is designed to identify. · When your control chart signals ... Dictionary👋Hi!PleaseSignin|Register. AssignableCause DefinitionofAssignableCause: «BacktoGlossaryIndexAssignablecause,alsoknownasaspecialcause,isoneofthetwotypesofvariationacontrolchartisdesignedtoidentify.Let’sdefinewhatanassignablecausevariationisandcontrastitwithcommoncausevariation.Wewillexplorehowtoknowifyourcontrolissignalinganassignablecauseandhowtoreactifitis. Overview:Whatisanassignablecause?  Acontrolchartidentifiestwodifferenttypesofvariation:commoncausevariation(randomvariationresultingfromyourprocesscomponentsor6Ms)andassignableorspecialcausevariation. Assignablecausevariationispresentwhenyourcontrolchartshowsplottedpointsoutsidethecontrollimitsoranon-randompatternofvariation.Sincespecialcausevariationisunexpectedandduetosomefactorotherthanrandomness,youshouldbeabletoassignareasonorcausetoit.  Whenyourcontrolchartsignalsassignablecausevariation,yourprocessvariableissaidtobeoutofcontrol,orunstable.AssignablecausevariationsignalscanbeidentifiedbyuseoftheWesternElectricrules,whichinclude:   Onepointoutsideoftheuppercontrollimitorlowercontrollimit   Atrendof6or7consecutivepointsincreasingordecreasing   Acycleorrepeatingpattern   Arunof8ormoreconsecutivepointsoneithersideoftheaverageorcenterline. Assignablecausevariationcanbeattributedtoadefect,fault,mistake,delay,breakdown,accident,and/orshortageintheprocess.Oritcanbearesultofsomeuniquecombinationoffactorscomingtogethertoactuallyimprovetheprocess.Whenassignablecausesarepresent,yourprocessisunpredictable.Theproperactionandresponseistosearchforandidentifythespecificassignablecause.Ifyourprocesswasimprovedasaresultofyourassignablecause,thenincorporateitsothatthecauseisretainedandimprovementmaintained.Ifyourprocesswasharmedbytheassignablecause,thenseektoeliminateit. 3benefitsofanassignablecause Assignablecausescanbegoodorbad.Theyaresignalsthatsomethingunexpectedhappened.Listentothesignal. 1.Signalssomethinghashappened  Specialorassignablecausevariationsignalsthatsomethingunexpectedandnon-randomhasoccurredinyourprocess. 2.Specificcause Byinvestigatingandidentifyingthespecificcauseofyoursignal,youcannarrowinonyournextstepsforbringingtheprocessbackintocontrol. 3.Canbecomecommoncausevariation  Goodnews!Youfoundthatyourassignablecauseforloweredproductionwasduetoapoweroutage.Unfortunately,youmaynotbeabletostoppoweroutagesinyourcommunity.Ifnothingisdone,yourassignablecausebecomesacommoncause.  Youmightnotbeabletostoppoweroutages,butcouldyouinstallaback-upgenerator?Then,ifthegeneratordoesn’tkickon,youwillhaveanassignablecauseyoucandosomethingabout. Whyisanassignablecauseimportanttounderstand?  Interpretingwhatanassignablecausetellsyouisimportanttounderstand.  Providesdirectionforaction Sinceanassignablecausecanbeasignalofsomethinggoodorbad,youneedtounderstandthedifferentactions.Don’tignorespecialorassignablecauses.  Noteveryunusualpointhasanassignablecause  Whileatyourfavoritecasino,youmaythrowapairofdiceatthecrapstable.Isthereanassignablecauseforthrowingan11ora10,orisitrandomvariation?No,youwouldexpecttheprocessofrollingafairpairofdicetoshow10sand11s.Whatabouta13?Thatwouldbeunexpectedandprobablytheresultofsomethingunusualhappeningwiththedice.Thesameistrueforyourprocess.Don’tassumeanassignablespecialcauseunlessyourcontrolchartsignalsit.  Usefulfordeterminingwhetheryourimprovementsworked Whenyouimprovetheprocess,yourcontrolchartshouldsendsignalsofspecialcausevariation—hopefullyintherightdirection.Ifyoucanlinkthatsignaltothespecificassignablecauseofyourimprovement,thenyouknowitworked.  Anindustryexampleofanassignablecause  Theaccountsreceivabledepartmentofaretailchainstartedtogetcomplaintsfromitscustomersaboutoverbilling.Fortunately,themanagerofthedepartmenthadparticipatedinthecompany’sLeanSixSigmatrainingandhadbeenusingacontrolchartforerrors. Uponcloserreview,shenoticedthaterrorsseemedtooccurmoreonFridaysthantherestoftheweek.Infact,thechartshowedthatalmosteveryFriday,thedatapointswereoutsidetheuppercontrollimit.Shewasconcernedthatnobodywasidentifyingthatasasignalofspecialcause. Sheputtogetherasmallteamofclerkstoidentifywhythiswashappeningandwhethertherewasanassignablereasonorcauseforit.TheassignablecausewasdeterminedtobetheextraworkloadonFridays.  Theteamrecommendedachangeinproceduretobetterbalancetheworkloadduringtheweek.Continuedmonitoringshowedtheproblemwasresolved.Shealsoheldanall-handsmeetingtodiscusstheimportanceofnotignoringsignalsofspecialcausevariationandtheneedtoseekoutanassignablecauseandtaketheappropriateaction. 3bestpracticeswhenthinkingaboutanassignablecause  Signalsofspecialcausevariationrequireyoutosearchforandidentifytheassignablecause. 1.Documentyoursearch  Ifyou’veidentifiedtheassignablecause,documenteverything.Ifthiscausehappensagaininthefuture,peoplewillhavesomebackgroundtoactquicklyandeliminate/incorporateanyactions. 2.Quicklyidentifythecause  Timeisoftheessence.Ifthecauseisresultinginadeterioratingprocess,actquicklytoidentifyandeliminatethecause.Therecommendationisthesameifyourcausemadetheprocessbetter,otherwise,whateverhappenedtoimprovetheprocesswillbelostastimegoesby. 3.Don’tignoresignalsofassignablecause  Evenifyougetasinglesignalofspecialcause,searchfortheassignablecause.Youmaychoosenottotakeanyactionintheeventitisafleetingcause,butatleasttrytoidentifytheassignablecause. FrequentlyAskedQuestions(FAQ)aboutanassignablecause 1.Isanassignablecausealwaysbad?  No.Itisanindicationthatsomethingunexpectedhappenedinyourprocess.Itcouldbeagoodorbadthing.Ineithercase,searchforandidentifytheassignablecauseandtaketheappropriateaction.  2.Whataresomesourcesofanassignablecause?  Somesourcesmaybeyourprocesscomponentssuchaspeople,methods,environment,equipment,materials,orinformation.Yourprocessvariationcancomefromtheseitemsandcanbetheassignablecauseofasignalofspecialcausevariation. 3.HowdoItellifIshouldlookforanassignablecause?  Controlchartsweredevelopedtodistinguishbetweencommonandspecialcausevariation.Iftheysignalspecialcausevariationinyourprocess,seekoutanassignablecauseandtaketheappropriateactionofeithereliminatingorincorporatingyourassignablecause. Finalthoughtsonanassignablecause Allprocesseswillexhibittwotypesofvariation.Commoncausevariationisrandom,expected,andaresultofvariationintheprocesscomponents.Specialcausevariationisnon-random,unexpected,andaresultofaspecificassignablecause.  Ifyougetasignalofspecialcausevariation,youneedtosearchforandidentifytheassignablecause.Oncefound,youwilleitherseektoincorporateoreliminatethecausedependingonwhetherthecauseimprovedorhurtyourprocess. 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