ASTM_E_2281_-_15.pdf

Designation:E228115An American National StandardStandard Practice forProcess Capability and Performance Measurement1This standard is issued under the fixed designation E2281;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon()indicates an editorial change since the last revision or reapproval.1.Scope1.1 This practice provides guidance for determining processcapability and performance under several common scenarios ofuse including:(a)normal distribution based capability andperformance indices such as Cp,Cpk,Pp,and Ppk;(b)processcapability using attribute data for non-conforming units andnon-conformities per unit type variables,and(c)additionalmethods in working with process capability or performance.2.Referenced Documents2.1 ASTM Standards:2E456 Terminology Relating to Quality and StatisticsE2334 Practice for Setting an Upper Confidence Bound Fora Fraction or Number of Non-Conforming items,or a Rateof Occurrence for Non-conformities,Using AttributeData,When There is a Zero Response in the Sample2.2 Other Document:MNL 7 Manual on Presentation of Data and Control ChartAnalysis33.Terminology3.1 DefinitionsUnless otherwise noted,all statisticalterms are defined in Terminology E456.3.1.1 long term standard deviation,LT,nsample standarddeviation of all individual(observed)values taken over a longperiod of time.3.1.1.1 DiscussionA long period of time may be definedas shifts,weeks,or months,etc.3.1.2 process capability,PC,nstatistical estimate of theoutcome of a characteristic from a process that has beendemonstrated to be in a state of statistical control.3.1.3 process capability index,Cp,nan index describingprocess capability in relation to specified tolerance.3.1.4 process performance,PP,nstatistical measure of theoutcome of a characteristic from a process that may not havebeen demonstrated to be in a state of statistical control.3.1.5 process performance index,Pp,nindex describingprocess performance in relation to specified tolerance.3.1.6 short term standard deviation,ST,nthe inherentvariation present when a process is operating in a state ofstatistical control,expressed in terms of standard deviation.3.1.6.1 DiscussionThis may also be stated as the inherentprocess variation.3.1.7 stable process,nprocess in a state of statisticalcontrol;process condition when all special causes of variationhave been removed.3.1.7.1 DiscussionObserved variation can then be attrib-uted to random(common)causes.Such a process will gener-ally behave as though the results are simple random samplesfrom the same population.3.1.7.2 DiscussionThis state does not imply that therandom variation is large or small,within or outside ofspecification,but rather that the variation is predictable usingstatistical techniques.3.1.7.3 DiscussionThe process capability of a stable pro-cess is usually improved by fundamental changes that reduceor remove some of the random causes present or adjusting themean towards the preferred value,or both.3.1.7.4 DiscussionContinual adjustment of a stable pro-cess will increase variation.3.2 Definitions of Terms Specific to This Standard:3.2.1 lower process capability index,Cpkl,nindex describ-ing process capability in relation to the lower specificationlimit.3.2.2 lower process performance index,Ppkl,nindex de-scribing process performance in relation to the lower specifi-cation limit.3.2.3 minimum process capability index,Cpk,nsmaller ofthe upper process capability index and the lower processcapability index.3.2.4 minimum process performance index,Ppk,nsmallerof the upper process performance index and the lower processperformance index.1This practice is under the jurisdiction ofASTM Committee E11 on Quality andStatistics and is the direct responsibility of Subcommittee E11.30 on StatisticalQuality Control.Current edition approved Oct.1,2015.Published October 2015.Originallyapproved in 2003.Last previous edition approved in 2012 as E2281 08a(2012)1.DOI:10.1520/E2281-15.2For referenced ASTM standards,visit the ASTM website,www.astm.org,orcontact ASTM Customer Service at serviceastm.org.For Annual Book of ASTMStandards volume information,refer to the standards Document Summary page onthe ASTM website.3Availablefrom ASTMHeadquarters,100BarrHarborDrive,W.Conshohocken,PA 19428.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 3.2.5 special cause,nvariation in a process coming fromsource(s)outside that which may be expected due to chancecauses(or random causes).3.2.5.1 DiscussionSometimes“special cause”is taken tobe synonymous with“assignable cause.”However,a distinc-tion should be recognized.A special cause is assignable onlywhen it is specifically identified.Also,a common cause may beassignable.3.2.5.2 DiscussionA special cause arises because of spe-cific circumstances which are not always