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1、Chapter14StatisticalQualityControlTheaiminproductionshouldbenotjusttogetstatisticalcontrol,buttoshrinkvari-ation.Costsgodownasvariationisreduced.Itisnotenoughtomeetspecifications.W.E.Deming(1982),OutoftheCrisis,MITPress,p.33414.1OrientationThefluctuationsinthequalityofamanufacturedproduct,suchasinte
2、gratedcircuits,steelbars,andcottonfabric,arearesultofthevariationsintherawmaterials,machines,andworkersusedtomanufacturethem.WemeasurequalitybyrecordingthevalueofanumericalvariableXdefinedonthepopulationofmanufacturedobjects.Examplesofsuchvariablesincludethethicknessofanepitaxiallayerdepositedonasi
3、liconwafer,thecompressivestrengthofconcrete,thetensilestrengthofasteelcable,etc.Sometimesthevariableiscategoricalaswhenweclassifyeachiteminspectedasconformingornon-conforming.InallcasestheproductisrequiredtomeetastandardusuallystatedintheformofloweranduppercontrollimitsonthevariableX.Theprimarygoa
4、lofaqualitycontrolprogramistosavemoneybyreducingoreliminatingtheproductionofdefectiveproductssince,ifuncorrected,thiscanonlyresultindecliningsales,lossofmarketshare,layoffs,andbankruptcy.ThefollowingexampleiscitedinKazmierski(1995),(StatisticalProblemSolvinginQualityEngineering,McGraw-Hill,NewYork)
5、:Asmallcompanyhad$10millioninsalesoveragivenperiod.Duringthatperiod,warrantycostswere$1,550,000.Scrapandreworkwere$150,000.Theinspectionbudgetwas$250,000,andthepreventionbudgetwas$50,000.Thetotalcostofpoorqualitywas$2million(20%ofsales).Reducingthecostofpoorqualityby50%wouldincreaseprofitby$1millio
6、n.Amongthemostimportanttechniquesfordetectingvariationinthequalityofamanufac-turedproductisthecontrolchart.Wewillstudyseveraltypesofcontrolchartsdependingonwhetherthevariablebeingmeasuredisnumericalorcategorical.Thexchartmoni-torsthefluctuationsaboutthetargetvalueµandtheRchartmonitorsthefluctuations
7、intheprocessvariation.Twootherchartsarethepchart,whichmonitorstheproportionofinspecteditemsrejectedasnon-conforming,andthecchart,whichmonitorsthenumberdefectsinanon-conformingitem.OrganizationofChapter1.Section14
