ASTM E2281-2008a(2012)e1 Standard Practice for Process and Measurement Capability Indices《过程和测量能力指数的标准实施规程》.pdf
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1、Designation: E2281 08a (Reapproved 2012)1An American National StandardStandard Practice forProcess and Measurement Capability Indices1This standard is issued under the fixed designation E2281; the number immediately following the designation indicates the year oforiginal adoption or, in the case of
2、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.1NOTEEditorial corrections were made to Equation 20 in October 2012.1. Scope1.1 This practice provides guida
3、nce for the use of capabilityindices for evaluating process capability and performance.Process capability indices compare the variability of a processquality measure against product specifications or tolerancesand assume the process is in a state of statistical control.Process performance indices ar
4、e useful in situations when theprocess is not in a state of statistical control.2. Referenced Documents2.1 ASTM Standards:2E456 Terminology Relating to Quality and StatisticsE2586 Practice for Calculating and Using Basic Statistics2.2 ISO Standard:ISO 3534-2 StatisticsVocabulary and Symbols-Statisti
5、calQuality Control32.3 Other Document:MNL 7 Manual on Presentation of Data and Control ChartAnalysis43. Terminology3.1 DefinitionsUnless otherwise noted, all statistical termsare defined in Terminology E456.3.2 Definitions of Terms Specific to This Standard:3.2.1 average standard deviation, s, narit
6、hmetic averageof sample standard deviations.3.2.2 long term standard deviation, LT,nsample standarddeviation of all individual (observed) values taken over a longperiod of time.3.2.2.1 DiscussionA long period of time may be definedas shifts, weeks, or months, etc.3.2.3 lower process capability index
7、, Cpkl,nindex describ-ing process capability in relation to the lower specificationlimit.3.2.4 lower process performance index, Ppkl,nindex de-scribing process performance in relation to the lower specifi-cation limit.3.2.5 minimum process capability index, Cpk,nsmaller ofthe upper process capabilit
8、y index and the lower processcapability index.3.2.6 minimum process performance index, Ppk,nsmallerof the upper process performance index and the lower processperformance index.3.2.7 process capability, PC, nstatistical estimate of theoutcome of a characteristic from a process that has beendemonstra
9、ted to be in a state of statistical control.3.2.8 process capability index, Cp,nan index describingprocess capability in relation to specified tolerance.3.2.9 process performance, PP, nstatistical measure of theoutcome of a characteristic from a process that may not havebeen demonstrated to be in a
10、state of statistical control.3.2.10 process performance index, Pp,nindex describingprocess performance in relation to specified tolerance.3.2.11 range, R, nmaximum value minus the minimumvalue in a sample. E25863.2.12 short term standard deviation, ST,nthe inherentvariation present when a process is
11、 operating in a state ofstatistical control, expressed in terms of standard deviation.3.2.12.1 DiscussionThis may also be stated as the inher-ent process variation.3.2.13 special cause, nsource of intermittent variation in aprocess. ISO 3534-23.2.13.1 DiscussionSometimes “special cause” is taken tob
12、e synonymous with “assignable cause.” However a distinctionshould be recognized. A special cause is assignable only whenit is specifically identified. Also a common cause may beassignable.3.2.13.2 DiscussionA special cause arises because of1This practice is under the jurisdiction ofASTM Committee E1
13、1 on Quality andStatistics and is the direct responsibility of Subcommittee E11.30 on StatisticalQuality Control.Current edition approved Oct. 1, 2012. Published October 2012. Originallyapproved in 2003. Last previous edition approved in 2008 as E2281 08a. DOI:10.1520/E2281-08AR12.2For referenced AS
14、TM 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.3Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4t
15、h Floor, New York, NY 10036, http:/www.ansi.org.4Available from ASTM Headquarters, 100 Barr Harbor Drive, W.Conshohocken, PA 19428.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1specific circumstances which are not always present. As
16、 such,in a process subject to special causes, the magnitude of thevariation from time to time is unpredictable.3.2.14 stable process, nprocess in a state of statisticalcontrol; process condition when all special causes of variationhave been removed. ISO 3534-23.2.14.1 DiscussionObserved variation ca
17、n 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.3.2.14.2 DiscussionThis state does not imply that therandom variation is large or small, within or outside ofspecification, but rather that t
18、he variation is predictable usingstatistical techniques.3.2.14.3 DiscussionThe process capability of a stableprocess is usually improved by fundamental changes thatreduce or remove some of the random causes present oradjusting the mean towards the preferred value, or both.3.2.14.4 DiscussionContinua
19、l adjustment of a stable pro-cess will increase variation.3.2.15 upper process capability index, Cpku,nindex de-scribing process capability in relation to the upper specificationlimit.3.2.16 upper process performance index (Ppku), nindexdescribing process performance in relation to the upper speci-f
20、ication limit.4. Significance and Use4.1 Process CapabilityProcess capability can be definedas the natural or inherent behavior of a stable process that is ina state of statistical control (1).5A “state of statistical control”is achieved when the process exhibits no detectable patterns ortrends, suc
21、h that the variation seen in the data is believed to berandom and inherent to the process. Process capability is linkedto the use of control charts and the state of statistical control.Aprocess must be studied to evaluate its state of control beforeevaluating process capability.4.2 Process ControlTh
22、ere are many ways to implementcontrol charts, but the most popular choice is to achieve a stateof statistical control for the process under study. Special causesare identified by a set of rules based on probability theory. Theprocess is investigated whenever the chart signals the occur-rence of spec
23、ial causes. Taking appropriate actions to eliminateidentified special causes and preventing their reappearance willultimately obtain a state of statistical control. In this state, aminimum level of variation may be reached, which is referredto as common cause or inherent variation. For the purpose o
24、fthis standard, this variation is a measure of the uniformity ofprocess output, typically a product characteristic.4.3 Process Capability IndicesThe behavior of a process(as related to inherent variability) in the state of statisticalcontrol is used to describe its capability. To compare a processwi
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