英文质量管理课件质量管理-Quality-Management-Stabilizing-and-Improving-a-Process-with-Control-Charts

Chapter6StabilizingandImprovingaProcesswithControlChartsChapter6

StabilizingandImprovingaProcesswithControlChartsSections

IntroductionProcessVariationTheStructureofControlChartsStabilizingaProcesswithControlChartsAdvantagesofaStableProcessImprovingaProcesswithControlChartsTwoPossibleMistakesinUsingControlChartsStabilizingaprocesswithControlChartsQualityConsciousnessandTypesofControlChartsThreeUsesofControlChartsTakeAwayKnowledgeSummaryKeyWordsReferencesExercisesAppendix6:UsingMinitabforp-chartsChapter6

StabilizingandImprovingaProcesswithControlChartsLearningObjectives(Youwillbeableto:)Toexplaintheneedforcontinualreductionofvariation,evenwhenthequalitycharacteristiciswithinspecifications.Todiscusstheuseofcontrolchartstostabilizeandimproveaprocess.Todiscusstheconsequencesofoverandunder-adjustmentofaprocess.Todetectout-of-controlbehaviorusingthe7WesternElectricrules.Todescribehowattributescontrolchartscanbeusedfordefectprevention.Todescribehowvariablescontrolchartscanbeusedfornever-endingimprovement.Todiscussthepurposesofstudyingcontrolcharts.

Introduction

Aprocessthathasbeendefinedanddocumentedcanbestabilizedandthenimproved.Ingreatmeasurethiscanbeaccomplishedthroughtheuseofstatisticalcontrolcharts,aswellasothertechniques.Withoutvalidmeasurements,processimprovementsaredifficultifnotimpossible,andperhapsthebestmeansofmeasuringprocessperformanceisastatisticalcontrolchart.Controlchartsandothertoolsandmethodswedescribemustbeusedinanenvironmentthatprovidesapositiveatmosphereforprocessimprovement;andtopmanagementmustsincerelydesirerealprocessimprovement.W.E.Demingpointsoutthat"anyattempttousestatisticaltechniquesunderconditionsthatrobthehourlyworkerofhisprideinworkmanshipwillleadtodisaster.“Withthiscaveat,wemaybegintoconsidertheissuesofstabilizingandimprovingadocumentedanddefinedprocess.ProcessVariationIntroductionRecall,wecanclassifyprocessvariationastheresultofeithercommoncausesorspecialcauses.Commonvariationsareinherentineveryprocess.Managementshouldnotholdworkersresponsibleforsuchsystemproblems;thesystemismanagement'sresponsibility.Variationscreatedbyspecialcauseslieoutsidethesystem.Frequentlytheirdetection,possibleavoidance,andrectificationaretheresponsibilityofthepeopledirectlyinvolvedwiththeprocess.ProcessVariationControlChartsandVariationControlchartsareusedtoidentifyanddifferentiatebetweencommonandspecialcausesofvariation.Whenaprocessnolongerexhibitsspecialvariation,butonlycommonvariation,itissaidtobestable.Whenonlycommoncausesofvariationarepresentinaprocess,managementmusttakeactiontoreducethedifferencebetweencustomerneedsandprocessperformancebyendeavoringtomovethecenterlineoftheprocessclosertoadesiredlevel(nominal)and/orbyreducingthemagnitudeofcommonvariation.ProcessVariationTheNeedfortheContinualReductionofVariationDemingwrote,"Itisgoodmanagementtoreducethevariationinanyqualitycharacteristic,whetherthischaracteristicbeinastateofcontrolornot,andevenwhenfewornodefectivesarebeingproduced."

DonaldJ.WheelerandDavidS.Chambersclarifytherationaleforthecontinualreductionofprocessvariation.Aprocesscanbedescribedasexistinginoneoffourstates:chaos,thebrinkofchaos,thethresholdstate,andtheidealstate.Whenaprocessisinastateofchaos,itisproducingsomenonconformingproductanditisnotinastateofstatisticalcontrol.Thereisnowaytoknoworpredictthepercentageofnonconformingproductthattheprocesswillgenerate.Aprocessonthebrinkofchaosproduces100percentconformingproduct;however,theprocessisnotstable.Hencethereisnoguaranteethattheprocesswillcontinuetoproduce100percentconformingproductindefinitely.Asitisunstable,theprocessmaywanderandtheproduct'scharacteristicsmaychangeatanytime,enteringastateofchaos.Thethresholdstatedescribesastableprocessthatproducessomenonconformingproduct;processvariationresultsfromcommoncausesthatareaninherentpartofthesystem.Theonlywaytoreducethisvariationistoimprovetheprocessitself.

Theidealstatedescribesastableprocessproducing100percentconformingproduct.Itisnotanaturalstate;forceswillalwaysexisttopushtheprocessawayfromtheidealstate.WheelerandChamberslikenthisphenomenontoentropy,inthatthereissimilarlyatrendtowarddisorderintheuniverse.TheStructureofControlChartsAllcontrolchartshaveacommonstructure.Theyhaveacenterline(representingtheprocessaverage)andupperandlowercontrollimits(called3-sigmalimits)thatprovideinformationontheprocessvariation.Controlchartsareconstructedbydrawingsamplesandtakingmeasurementsofaprocesscharacteristic.Eachsetofmeasurementsiscalledasubgroup.Controllimitsarebasedonthevariationthatoccurswithinthesampledsubgroups.Inthisway,variationbetweenthesubgroupsisintentionallyexcludedfromthecomputationofthecontrollimits;thecommonprocessvariationbecomesthevariationonwhichwecalculatethecontrollimits.Thecontrollimitcomputationsassumethattherearenospecialcausesofvariationaffectingtheprocess.Ifaspecialcauseofvariationispresent,thecontrolchart,basedsolelyoncommonvariation,willhighlightwhenandwherethespecialcauseoccurred.Whenthedataconsistofaseriesoffractionsthataredefectiveorpossesssomeothercharacteristicofinterest,theappropriatecontrolchartisap-chart.Thisisadepictionoftheprocessoutputintermsofanattributeofinterest-inourexample,thefractiondefective.Thecenterlineforap-chartisthemeanofthefractiondefective.Controllimitsarecalculatedasplusandminusthreetimesthestandarderror.Thestandarderrorfortheaverageproportionis:Wherenisthesubgroupsize.Usingthisvalue,theupperandlowercontrollimitsforap-chartaregivenby:

Wecannowfindthenumericalvaluesforconstructingourp-chart:Noticethatsinceanegativefractiondefectiveisnotpossible,thelowercontrollimitissetat0.00.

UnknownNewUntrainedOperatorTerminalMalfunction(Replacedwith“Old〞StandbyUnit)Theactiontakenontheprocessstemmingfrominvestigationsofdays8and22shouldchangetheprocesssothatthespecialcausesofvariationwillbeeliminated.Consequently,thedatafromdays8and22maynowbedeleted.Aftereliminatingthedataforthedaysinwhichthespecialcausesofvariationarefound,thecontrolchartstatisticsarerecomputed.AdvantagesofaStableProcessAstableprocessisaprocessthatexhibitsonlycommonvariationorvariationresultingfrominherentsystemlimitations.Theadvantagesofachievingastableprocessare:Managementknowstheprocesscapabilityandcanpredictperformance,costs,andqualitylevels.Productivitywillbeatamaximum,andcostswillbeminimized.Managementwillbeabletomeasuretheeffectsofchangesinthesystemwithgreaterspeedandreliability.Ifmanagementwantstoalterspecificationlimits,itwillhavethedatatobackupitsdecision.Astableprocessisabasicrequirementforprocessimprovementefforts.ImprovingaProcesswithControlChartsOnceaprocessisstable,ithasaknowncapability.Astableprocessmay,nevertheless,produceanunacceptablenumberofdefects(thresholdstate)Therearetwoareasforactiontoreducethedifferencebetweencustomerneedsandprocessperformance.First,actionmaybetakentochangetheprocessaverage.Second,managementcanacttoreducethelevelofcommonvariationwithaneyetowardnever-endingimprovementoftheprocess.Theworkerscanonlysuggestchanges;theycannoteffectchangestothesystem.TwoPossibleMistakesinUsingControlChartsTherearetwotypesofmistakesthattheuserofacontrolchartmaymake:over-adjustmentandunder-adjustment.Properuseofcontrolchartswillminimizethetotaleconomicconsequencesofmakingeitherofthesetypesoferrors.Over-adjustmentTheover-adjustmenterroroccurswhentheuserreactstoswingsintheprocessdatathataremerelytheresultofcommonvariation,suchasadjustingaprocessdownwardifitspastoutputisaboveaverageoradjustingaprocessupwardifitspastoutputisbelowaverage.Whenaprocessisover-adjusted,itresemblesacarbeingover-steered,veeringbackandforthacrossthehighway.Ingeneral,processesshouldbeadjusted--notonthebasisoftime-to-timeobservations--butonthebasisofinformationprovidedbyastatisticalcontrolchart.Under-adjustmentUnder-adjustment,orlackofattention,resultswhenaprocessisoutofcontrolandnoeffortismadetoprovidethenecessaryregulation.Theprocessswingsupanddowninresponsetooneormorespecialcausesofvariation,whichmayhavecompoundingeffects.Avoidingbothofthesemistakesallofthetimeisanimpossibletask.Neveradjustingtheprocess-sothatwenevermakethemistakeofover-adjusting-couldresultinsevereunder-adjustment.Ontheotherhand,ifwemadeveryfrequentadjustmentstoavoidtheproblemofunder-adjustment,wewouldprobablybeover-adjusting.Controlchartsprovideaneconomicalmeanstominimizethetotallossthatresultsfromthesetwoerrors.Consequently,controlchartsprovidemanagementwithinformationonwhentotakeactiononaprocessandwhentoleaveitalone.SomeOut-of-ControlEvidenceWeknowthataprocessexhibitsalackofstatisticalcontrolifasubgroupstatisticfallsbeyondeitherofthecontrollimits.Butitispossibleforallsubgroupstatisticstobewithinthecontrollimitswhilethereareotherfactorsthatindicatealackofcontrolintheprocess.Stableprocessesalwaysexhibitrandompatternsofvariation.Accordingly,mostdatapointswilltendtoclusteraboutthemeanvalue,orcenterline,withanapproximatelyequalnumberofpointsfallingaboveandbelowthemean.Afewofthevalueswilllieclosetothecontrollimits.Pointswillrarelyfallbeyondacontrollimit.Alsotherewillseldombeprolongedrunsupwardordownwardforanumberofsubgroups.Ifoneormoreoftheseconditionsisviolatedinacontrolchart,thechartdoesnotexhibitstatisticalcontrol.Hence,foraprocessthatisoutofcontrol,therewillbeanabsenceofpointsnearthecenterline,anabsenceofpointsnearthecontrollimits,oneormorepointslocatedbeyondthecontrollimits,orrunsornonrandompatternsamongthepoints.Sothatwecanexaminepatternsindicatingalackofcontrol,theareabetweenthecontrollimitsisdividedintosixbands,eachbandonestandarderrorwide.Asbelow,bandswithinonestandarderrorofthecenterlinearecalledtheCzones;bandsbetweenoneandtwostandarderrorsfromthecenterlinearecalledBzones;andtheoutermostbands,whichliebetweentwoandthreestandarderrorsfromthemean,areAzones.Sevensimplerulesbasedonthesebandsarecommonlyappliedtodetermineifaprocessisexhibitingalackofstatisticalcontrol.Anyout-of-controlpointsfoundaremarkedwithanXdirectlyonthecontrolchart.Rule1.Aprocessexhibitsalackofcontrolifanysubgroupstatisticfallsoutsideofthecontrollimits.

Aswehavealreadyseen,thisisthefirstcriterion--andthemostobviousone.Rule2.AprocessexhibitsalackofcontrolifanytwooutofthreeconsecutivesubgroupstatisticsfallinoneoftheAzonesorbeyondonthesamesideofthecenterline.Rule3.AprocessexhibitsalackofcontroliffouroutoffiveconsecutivesubgroupstatisticsfallinoneoftheBzonesorbeyondonthesamesideofthecenterline.Rule4.Aprocessexhibitsalackofcontrolifeightormoreconsecutivesubgroupstatisticslieonthesamesideofthecenterline.Rule5.Aprocessexhibitsalackofcontrolifeightormoreconsecutivesubgroupstatisticsmoveupwardinvalueorifeightormoreconsecutivesubgroupstatisticsmovedownwardinvalue.Rule6.Aprocessexhibitsalackofcontrolifanunusuallysmallnumberofrunsaboveandbelowthecenterlinearepresent(asaw-toothpattern).Rule7.Aprocessexhibitsalackofcontrolif13consecutivepointsfallwithinzoneConeithersideofthecenterline.QualityConsciousnessandTypesofControlChartsNoqualityconsciousnessDefectdetectionDefectpreventionNever-endingimprovementInnovationsNoQualityConsciousness:AcceptEverythingwithoutQuestionThelowestlevelofqualityconsciousnessistoaccepteverythingfromavendor,withoutquestion.Thistypeofqualityconsciousnessisborneoutofalackofawarenessofquality,orpastsatisfactionandaconsequentlackofconcernaboutpoorquality.DefectDetection:MassInspectiontoSortConformingandNon-ConformingOutputThepurposeofdefectdetectionistosortconformingandnonconformingproductsorservicesthroughmassinspection(Deming'sPoint3).Defectdetectionassumesthatdefectswillbeproduced;theyareexpectedandanticipated.Inthisstageofqualityconsciousness,nofeedbackloopsortoolsareavailableforcorrectingthefactorsthatcreatedthedefectivesinthefirstplace.DefectPrevention:AttributeControlChartsThepurposeofdefectpreventionistoachieve"zerodefects."Thisstageofqualityconsciousnessassumesthatifallproductsandservicesarewithinspecificationlimits,thenalloutputwillmeetcustomers'needsandwants.Thisisthegoalpostviewofquality.Theinitialentryintodefectpreventiongenerallyinvolvestheuseofcontrolchartsbasedonattributedata,suchasconformingversusnonconformingwithrespecttosomespecificationlimit.DefectPrevention:AttributeControlChartsAttributecontrolchartscanhelpmoveprocessestowardazero