>> P.496
HistogramAdiagramthatdividestherangebetweenthemaximumandtheminimummeasurementvaluesintoseveraldivisionsandshowsthenumberofvalues(appearancefrequency)ineachdivisionintheformofabargraph.Thismakesiteasiertounderstandtheroughaverageortheapproximateextentofdispersion.Abell-shapedsymmetricdistributioniscalledthenormaldistributionandismuchusedintheoreticalexamplesonaccountofitseasilycalculablecharacteristics.However,cautionshouldbeobservedbecausemanyrealprocessesdonotconformtothenormaldistribution,anderrorwillresultifitisassumedthattheydo.ProcesscapabilityProcess-specicperformancedemonstratedwhentheprocessissufcientlystandardized,anycausesofmalfunctionsareeliminated,andtheprocessisinastateofstatisticalcontrol.Theprocesscapabilityisrepresentedbymean±3or6whenthequalitycharacteristicoutputfromtheprocessshowsnormaldistribution.(sigma)indicatesstandarddeviation.Processcapabilityindex(PCIorCp)Theindexvalueiscalculatedbydividingthetoleranceofatargetcharacteristicbytheprocesscapability(6betweenthemean(X)andthestandardvalueby3representthisindexincasesofaunilateraltolerance.Theprocesscapabilityindexassumesthatacharacteristicfollowsthenormaldistribution.).ThevaluecalculatedbydividingthedifferencemaybeusedtoNote:Ifacharacteristicfollowsthenormaldistribution,99.74%dataiswithintherange±3fromthemean.BilateraltoleranceUSL-LSLCp=—6USL:UpperspecicationlimitLSL:LowerspecicationlimitUnilateraltolerance...IfonlytheupperlimitisstipulatedUSL-XCp=—3Unilateraltolerance...IfonlythelowerlimitisstipulatedCp=—X-LSL3Qualitycontrol(QC)Asystemforeconomicallyproducingproductsorservicesofaqualitythatmeetscustomerrequirements.ProcessqualitycontrolActivitiestoreducevariationinproductoutputbyaprocessandkeepthisvariationlow.Processimprovementandstandardizationaswellastechnologyaccumulationarepromotedthroughtheseactivities.Statisticalprocesscontrol(SPC)Processqualitycontrolthroughstatisticalmethods.PopulationAgroupofallitemsthathavecharacteristicstobeconsideredforimprovingandcontrollingprocessesandqualityofproduct.Agroupwhichistreatedbasedonsamplesisusuallythepopulationrepresentedbythesamples.LotCollectionofproductproducedunderthesameconditions.SampleAnitemofproduct(oritems)takenoutofthepopulationtoinvestigateitscharacteristics.SamplesizeNumberofproductitemsinthesample.BiasValuecalculatedbysubtractingthetruevaluefromthemeanofmeasurementvalueswhenmultiplemeasurementsareperformed.Dispersion,ImprecisionVariationinthevaluesofatargetcharacteristicinrelationtothemeanvalue.Standarddeviationisusuallyusedtorepresentthedispersionofvaluesaroundthemean.Upperspecificationlimit(USL)MeanTargetprocessmeanLowerspecificationlimit(LSL)FrequencyNoncomplianceMeasurementvaluesBiasDispersion/imprecisionTolerance(USL-LSL)09-31QuickGuidetoPrecisionMeasuringInstrumentsQualityControl
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