1、Designation: E177 10An American National StandardStandard Practice forUse of the Terms Precision and Bias in ASTM Test Methods1This standard is issued under the fixed designation E177; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision
2、, 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.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 The purpose of this pra
3、ctice is to present conceptsnecessary to the understanding of the terms “precision” and“bias” as used in quantitative test methods. This practice alsodescribes methods of expressing precision and bias and, in afinal section, gives examples of how statements on precisionand bias may be written for AS
4、TM test methods.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory requirements prior to us
5、e.2. Referenced Documents2.1 ASTM Standards:2E178 Practice for Dealing With Outlying ObservationsE456 Terminology Relating to Quality and StatisticsE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test MethodE2282 Guide for Defining the Test Result of a Test Metho
6、d3. Terminology3.1 DefintionsTerminology E456 provides a more exten-sive list of terms in E11 standards.3.1.1 accepted reference value, na value that serves as anagreed-upon reference for comparison, and which is derivedas: (1) a theoretical or established value, based on scientificprinciples, (2) a
7、n assigned or certified value, based on experi-mental work of some national or international organization, or(3) a consensus or certified value, based on collaborativeexperimental work under the auspices of a scientific orengineering group.3.1.1.1 DiscussionA national or international organiza-tion,
8、 referred to in 3.1.1 (2), generally maintains measurementstandards to which the reference values obtained are traceable.3.1.2 accuracy, nthe closeness of agreement between atest result and an accepted reference value.3.1.2.1 DiscussionThe term accuracy, when applied to aset of test results, involve
9、s a combination of a randomcomponent and of a common systematic error or bias compo-nent.3.1.3 bias, nthe difference between the expectation of thetest results and an accepted reference value.3.1.3.1 DiscussionBias is the total systematic error ascontrasted to random error. There may be one or mores
10、ystematic error components contributing to the bias. A largersystematic difference from the accepted reference value isreflected by a larger bias value.3.1.4 characteristic, na property of items in a sample orpopulation which, when measured, counted or otherwise ob-served, helps to distinguish betwe
11、en the items. E22823.1.5 intermediate precision, nthe closeness of agreementbetween test results obtained under specified intermediateprecision conditions.3.1.5.1 DiscussionThe specific measure and the specificconditions must be specified for each intermediate measure ofprecision; thus, “standard de
12、viation of test results amongoperators in a laboratory,” or “day-to-day standard deviationwithin a laboratory for the same operator.”3.1.5.2 DiscussionBecause the training of operators, theagreement of different pieces of equipment in the samelaboratory and the variation of environmental conditions
13、withlonger time intervals all depend on the degree of within-laboratory control, the intermediate measures of precision arelikely to vary appreciably from laboratory to laboratory. Thus,intermediate precisions may be more characteristic of indi-vidual laboratories than of the test method.3.1.6 inter
14、mediate precision conditions, nconditions un-der which test results are obtained with the same test methodusing test units or test specimens taken at random from a singlequantity of material that is as nearly homogeneous as possible,and with changing conditions such as operator, measuringequipment,
15、location within the laboratory, and time.1This practice is under the jurisdiction of ASTM Committee E11 on Quality andStatistics and is the direct responsibility of Subcommittee E11.20 on Test MethodEvaluation and Quality Control.Current edition approved Oct. 1, 2010. Published November 2010. Origin
16、allyapproved in 1961. Last previous edition approved in 2008 as E177 08. DOI:10.1520/E0177-10.2For 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
17、 Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.7 observation, nthe process of obtaining informationregarding the presence or absence of an attribute of a testspecimen, or of making a reading on a
18、 characteristic ordimension of a test specimen. E22823.1.8 observed value, nthe value obtained by making anobservation. E22823.1.9 precision, nthe closeness of agreement betweenindependent test results obtained under stipulated conditions.3.1.9.1 DiscussionPrecision depends on random errorsand does
19、not relate to the accepted reference value.3.1.9.2 DiscussionThe measure of precision usually isexpressed in terms of imprecision and computed as a standarddeviation of the test results. Less precision is reflected by alarger standard deviation.3.1.9.3 Discussion“Independent test results” means re-s
20、ults obtained in a manner not influenced by any previousresult on the same or similar test object. Quantitative measuresof precision depend critically on the stipulated conditions.Repeatability and reproducibility conditions are particular setsof extreme stipulated conditions.3.1.10 repeatability, n
21、precision under repeatability con-ditions.3.1.10.1 DiscussionRepeatability is one of the conceptsor categories of the precision of a test method.3.1.10.2 DiscussionMeasures of repeatability defined inthis compilation are repeatability standard deviation and re-peatability limit.3.1.11 repeatability
22、conditions, nconditions where inde-pendent test results are obtained with the same method onidentical test items in the same laboratory by the same operatorusing the same equipment within short intervals of time.3.1.11.1 DiscussionSee precision, The “same operator,same equipment” requirement means t
23、hat for a particular stepin the measurement process, the same combination of operatorand equipment is used for every test result. Thus, one operatormay prepare the test specimens, a second measure the dimen-sions and a third measure the mass in a test method fordetermining density.3.1.11.2 Discussio
24、nBy “in the shortest practical period oftime” is meant that the test results, at least for one material, areobtained in a time period not less than in normal testing and notso long as to permit significant change in test material,equipment or environment.3.1.12 repeatability limit (r), nthe value be
25、low which theabsolute difference between two individual test results obtainedunder repeatability conditions may be expected to occur with aprobability of approximately 0.95 (95 %).3.1.12.1 DiscussionThe repeatability limit is 2.8 (1.96=2 ) times the repeatability standard deviation. This multi-plier
26、 is independent of the size of the interlaboratory study.3.1.12.2 DiscussionThe approximation to 0.95 is reason-ably good (say 0.90 to 0.98) when many laboratories (30 ormore) are involved, but is likely to be poor when fewer thaneight laboratories are studied.3.1.13 repeatability standard deviation
27、 (sr), nthe standarddeviation of test results obtained under repeatability condi-tions.3.1.13.1 DiscussionIt is a measure of the dispersion of thedistribution of test results under repeatability conditions.3.1.13.2 DiscussionSimilarly, “repeatability variance”and “repeatability coefficient of variat
28、ion” could be defined andused as measures of the dispersion of test results underrepeatability conditions.In an interlaboratory study, this isthe pooled standard deviation of test results obtained underrepeatability conditions.3.1.13.3 DiscussionThe repeatability standard deviation,usually considere
29、d a property of the test method, will generallybe smaller than the within-laboratory standard deviation. (Seewithin-laboratory standard deviation.)3.1.14 reproducibility, nprecision under reproducibilityconditions.3.1.15 reproducibility conditions, nconditions where testresults are obtained with the
30、 same method on identical testitems in different laboratories with different operators usingdifferent equipment.3.1.15.1 DiscussionIdentical material means either thesame test units or test specimens are tested by all thelaboratories as for a nondestructive test or test units or testspecimens are ta
31、ken at random from a single quantity ofmaterial that is as nearly homogeneous as possible.A different laboratory of necessity means a different operator,different equipment, and different location and under different super-visory control.3.1.16 reproducibility limit (R), nthe value below whichthe ab
32、solute difference between two test results obtained underreproducibility conditions may be expected to occur with aprobability of approximately 0.95 (95 %).3.1.16.1 DiscussionThe reproducibility limit is 2.8(1.96=2 ) times the reproducibility standard deviation. Themultiplier is independent of the s
33、ize of the interlaboratorystudy (that is, of the number of laboratories participating).3.1.16.2 DiscussionThe approximation to 0.95 is reason-ably good (say 0.90 to 0.98) when many laboratories (30 ormore) are involved but is likely to be poor when fewer thaneight laboratories are studied.3.1.17 rep
34、roducibility standard deviation (sR), nthe stan-dard deviation of test results obtained under reproducibilityconditions.3.1.17.1 DiscussionOther measures of the dispersion oftest results obtained under reproducibility conditions are the“reproducibility variance” and the “reproducibility coefficiento
35、f variation.”3.1.17.2 DiscussionThe reproducibility standard devia-tion includes, in addition to between-laboratory variability, therepeatability standard deviation and a contribution from theinteraction of laboratory factors (that is, differences betweenoperators, equipment and environments) with m
36、aterial factors(that is, the differences between properties of the materialsother than that property of interest).3.1.18 test determination, nthe value of a characteristic ordimension of a single test specimen derived from one or moreobserved values. E22823.1.19 test method, na definitive procedure
37、that producesa test result. E22823.1.20 test observation, nsee observation. E2282E177 1023.1.21 test result, nthe value of a characteristic obtainedby carrying out a specified test method. E22823.1.22 test specimen, nthe portion of a test unit needed toobtain a single test determination. E22823.1.23
38、 test unit, nthe total quantity of material (containingone or more test specimens) needed to obtain a test result asspecified in the test method. See test result. E22823.1.24 trueness, nthe closeness of agreement between thepopulation mean of the measurements or test results and theaccepted referenc
39、e value.3.1.24.1 Discussion“Population mean” is, conceptually,the average value of an indefinitely large number of test results3.1.25 within-laboratory standard deviation, nthe stan-dard deviation of test results obtained within a laboratory for asingle material under conditions that may include suc
40、h ele-ments as different operators, equipment, and longer timeintervals.3.1.25.1 DiscussionBecause the training of operators, theagreement of different pieces of equipment in the samelaboratory and the variation of environmental conditions withlonger time intervals depend on the degree of within-lab
41、oratorycontrol, the within-laboratory standard deviation is likely tovary appreciably from laboratory to laboratory.4. Significance and Use4.1 Part A of the “Blue Book,” Form and Style for ASTMStandards, requires that all test methods include statements ofprecision and bias. This practice discusses
42、these two conceptsand provides guidance for their use in statements about testmethods.4.2 PrecisionA statement of precision allows potentialusers of a test method to assess in general terms the testmethods usefulness with respect to variability in proposedapplications. A statement of precision is no
43、t intended to exhibitvalues that can be exactly duplicated in every users laboratory.Instead, the statement provides guidelines as to the magnitudeof variability that can be expected between test results whenthe method is used in one, or in two or more, reasonablycompetent laboratories. For a discus
44、sion of precision, see 8.1.4.3 BiasA statement of bias furnishes guidelines on therelationship between a set of typical test results produced bythe test method under specific test conditions and a related setof accepted reference values (see 9.1).4.3.1 An alternative term for bias is trueness, which
45、 has apositive connotation, in that greater bias is associated with lessfavorable trueness. Trueness is the systematic component ofaccuracy.4.4 AccuracyThe term “accuracy,” used in earlier editionsof Practice E177, embraces both precision and bias (see 9.3 andNote 3).5. Test Method5.1 Section 2 of t
46、he ASTM Regulations describes a testmethod as “a definitive procedure for the identification, mea-surement, and evaluation of one or more qualities, character-istics, or properties of a material, product, system or servicethat produces a test result.”5.2 In this practice only quantitative test metho
47、ds thatproduce numerical results are considered. Also, the word“material” is used to mean material, product, system or service;the word “property” is used herein to mean that a quantitativetest result can be obtained that describes a characteristic or aquality, or some other aspect of the material;
48、and “test method”refers to both the document and the procedure describedtherein for obtaining a quantitative test result for one property.For a discussion of test result, see 7.1.5.3 A well-written test method specifies control over suchfactors as the test equipment, the test environment, the quali-
49、fications of the operator (explicitly or implicitly), the prepara-tion of test specimens, and the operating procedure for usingthe equipment in the test environment to measure someproperty of the test specimens. The test method will alsospecify the number of test specimens required and howmeasurements on them are to be combined to provide a testresult (7.1), and might also reference a sampling procedureappropriate for the intended use of the method.5.4 It is necessary that the writers of the test method provideinstructions or
