PROCESSCAPABILITY
Thereare4basicmeasuresofprocesscapability:
Cpwhichindicateshowgoodtheprocesscouldbe.Itisashort-termmeasure.
Cpkwhichtakesintoaccountprocesscentringandindicateshowtheprocessisactuallyperforming.
PpwhichistheequivalentofCpforlongtermdata.
PpkwhichistheequivalentofCpkforlongtermdata.
ShortTermvs.Long-TermData
AsanexampleconsideraprocesscapabilitystudywherebyaQualityEngineerhastakenasampleof50armatureshaftsfromoneshiftandhascalculatedthecapabilityoftheprocess.Thiscalculationwillbeshorttermonlybecauselongtermdatawillneedtoinclude,forexample,everyshift,allmachineoperators,differentsetters,supervisors,materialbatchesetc.Soalong-termstudymaytakeseveraldaysasopposedtoashort-termstudywhichmayonlytakeseveralhours.
Intheeventthatshort-termdataonlyisavailablesixsigmamethodologyrecommendsthatweassumea1.5sigmashift.
DiagrammaticallyCpandCpkcanbeshownasfollows:
Exampleofaprocesscapabilitystudyusingminitab
From:capability.mtw).Goto”DataFiles"ontheTools/Metrics
pageofthiswebsiteandgetfile"capability.mtw"totryit.
STAT>QUALITYTOOLS>CAPABILITYANALYSIS(normal)
ResultsandInterpretation
Wecanseefromthehistogramthattheprocessiscapableofmeetingthespecification,i.e.lookatthewidthofthespeclimitsVsthewidthofthehistogram.ComparingCpandCpkalsoshowsthis,i.e.Cpis1.81andCpkis1.05.Theshorttermpredictedppmis713,whichmeansthisiswhatweshouldaimtogetourprocesstobycentringtheprocessandremovinglongtermvariability.PpandPpkrepresentthelong-termpositionandwecanseethatthepredictedlong-termppmis3966andwemustrememberthisiswhatthecustomerwouldreceive.Sothefirstpartoftheanalysisfortheabovewouldbetoinvestigatehowwecancentretheprocess.
UpperSpec.limit(USL)
LowerSpec.limit(LSL)
Thisprocessiscentred&wellwithinthespecification.
Cp=Cpkandisabout2.0
LowerSpec.limit(LSL)
UpperSpec.limit(USL)
OutsideUSL&LSL,defects
beingproduced
Althoughtheprocessisstillcentred,thistimetheprocessisoutsidethespec.limitsanddefectsarebeingproduced.
ForthisprocessCp=Cpkbutwillbe<1.0
LowerSpec.limit(LSL)
UpperSpec.limit(USL)
OutsideUSLsodefectsarebeingproduced
Thistimealthoughtheprocessisactuallycapable,i.e.Cp=2.0,theprocessisoffcentreandisproducingdefects,i.e.Cpk<1.0
OutsideLSL,defects
beingproduced
LowerSpec.limit(LSL)
UpperSpec.limit(USL)
Thisistheworstofbothworlds.Theprocessisneithercapablenorcentred.Cp&Cpkwillbothbe<1.0andCpk
Data&subgroupsize
Specificationlimits
CP=1.81
Cpk=1.05
Pp=1.5
Ppk=0.88
Predictedppmshortterm=713
Predictedppmlongterm=3966
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