ASTM_E_177_-_14.pdf

Designation:E17714An 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,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 U.S.Department of Defense.1.Scope1.1 The purpose of this practice 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 ASTM 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 use.2.Referenced Documents2.1 ASTM Standards:2E456 Terminology Relating to Quality and StatisticsE691 Practice for Conducting an Interlaboratory Study toDetermine the Precision of a Test MethodE1488 Guide for Statistical Procedures to Use in Developingand Applying Test MethodsE2282 Guide for Defining the Test Result of a Test MethodE2586 Practice for Calculating and Using Basic StatisticsE2587 Practice for Use of Control Charts in StatisticalProcess Control3.Terminology3.1 DefinitionsTerminology 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)an 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 DiscussionAnationalorinternationalorganization,referred to in 3.1.1(2),generally maintainsmeasurement standards to which the reference values obtainedare 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,involves 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 moresystematic 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 otherwiseobserved,helps to distinguish between the items.E22823.1.5coeffcient of variation,CV,nfor a nonnegativecharacteristic,the ratio of the standard deviation to the meanfor a population or sample.E25863.1.6 intermediate precision,nthe closeness of agreementbetween test results obtained under specified intermediateprecision conditions.3.1.6.1 DiscussionThe specific measure and the specificconditions must be specified for each intermediate measure ofprecision;thus,“standard deviation of test results amongoperators in a laboratory,”or“day-to-day standard deviationwithin a laboratory for the same operator.”3.1.6.2 DiscussionBecause the training of operators,theagreement of different pieces of equipment in the samelaboratory and the variation of environmental conditions 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.1This practice is under the jurisdiction ofASTM Committee E11 on Quality andStatistics and is the direct responsibility of Subcommittee E11.20 on Test MethodEvaluation and Quality Control.Current edition approved May 1,2014.Published May 2014.Originallyapproved in 1961.Last previous edition approved in 2013 as E177 13.DOI:10.1520/E0177-14.2For 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.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 3.1.7 intermediate 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 single