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ASTM E691-14

(Practice)

Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method

Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method

ABSTRACT
The procedure presented in this practice consists of three basic steps: planning the interlaboratory study, guiding the testing phase of the study, and analyzing the test result data. The analysis utilizes tabular, graphical, and statistical diagnostic tools for evaluating the consistency of the data so that unusual values may be detected and investigated, and also includes the calculation of the numerical measures of precision of the test method pertaining to both within-laboratory repeatability and between-laboratory reproducibility.
Tests performed on presumably identical materials in presumably identical circumstances do not, in general, yield identical results. This is attributed to unavoidable random errors inherent in every test procedure; the factors that may influence the outcome of a test cannot all be completely controlled. In the practical interpretation of test data, this inherent variability has to be taken into account. For instance, the difference between a test result and some specified value may be within that which can be expected due to unavoidable random errors, in which case a real deviation from the specified value has not been demonstrated. Similarly, the difference between test results from two batches of material will not indicate a fundamental quality difference if the difference is no more than can be attributed to inherent variability in the test procedure. Many different factors (apart from random variations between supposedly identical specimens) may contribute to the variability in application of a test method, including: a the operator, b equipment used, c calibration of the equipment, and d environment (temperature, humidity, air pollution, etc.). It is considered that changing laboratories changes each of the above factors. The variability between test results obtained by different operators or with different equipment will usually be greater than between test results obtained by a single operator using the same equipment. The variability between test results taken over a long period of time even by the same operator will usually be greater than that obtained over a short period of time because of the greater possibility of changes in each of the above factors, especially the environment.
The general term for expressing the closeness of test results to the “true” value or the accepted reference value is accuracy. To be of practical value, standard procedures are required for determining the accuracy of a test method, both in terms of its bias and in terms of its precision. This practice provides a standard procedure for determining the precision of a test method. Precision, when evaluating test methods, is expressed in terms of two measurement concepts, repeatability and reproducibility. Under repeatability conditions the factors listed above are kept or remain reasonably constant and usually contribute only minimally to the variability. Under reproducibility conditions the factors are generally different (that is, they change from laboratory to laboratory) and usually contribute appreciably to the variability of test results. Thus, repeatability and reproducibility are two practical extremes of precision.
The repeatability measure, by excluding the factors a through d as contributing variables, is not intended as a mechanism for verifying the ability of a laboratory to maintain“ in-control” conditions for routine operational factors such as operator-to-operator and equipment differences or any effects of longer time intervals between test results. Such a control study is a separate issue for each laboratory to consider for itself, and is not a recommended part of an interlaboratory study.
The reproducibility measure (including the factors a through d as sources of variability) reflects what precision might be expected when random portions of a homogeneous sample are sent to random “in-control” laboratories.
To obtain reasonable estimates of repeatability and reprod...

General Information

Status
Historical
Publication Date
31-Mar-2014
ICS
19.020 - Test conditions and procedures in general
Technical Committee
E11 - Quality and Statistics
Drafting Committee
E11.20 - Test Method Evaluation and Quality Control
Current Stage
Ref Project

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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E691 − 14 AnAmerican National Standard
Standard Practice for
Conducting an Interlaboratory Study to Determine the
1
Precision of a Test Method
This standard is issued under the fixed designation E691; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript 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.
INTRODUCTION
Testsperformedonpresumablyidenticalmaterialsinpresumablyidenticalcircumstancesdonot,in
general, yield identical results. This is attributed to unavoidable random errors inherent in every test
procedure; the factors that may influence the outcome of a test cannot all be completely controlled. In
the practical interpretation of test data, this inherent variability has to be taken into account. For
instance, the difference between a test result and some specified value may be within that which can
beexpectedduetounavoidablerandomerrors,inwhichcasearealdeviationfromthespecifiedvalue
has not been demonstrated. Similarly, the difference between test results from two batches of material
will not indicate a fundamental quality difference if the difference is no more than can be attributed
to inherent variability in the test procedure. Many different factors (apart from random variations
between supposedly identical specimens) may contribute to the variability in application of a test
method, including: a the operator, b equipment used, c calibration of the equipment, and d
environment (temperature, humidity, air pollution, etc.). It is considered that changing laboratories
changes each of the above factors.The variability between test results obtained by different operators
or with different equipment will usually be greater than between test results obtained by a single
operator using the same equipment. The variability between test results taken over a long period of
time even by the same operator will usually be greater than that obtained over a short period of time
because of the greater possibility of changes in each of the above factors, especially the environment.
The general term for expressing the closeness of test results to the “true” value or the accepted
referencevalueisaccuracy.Tobeofpracticalvalue,standardproceduresarerequiredfordetermining
the accuracy of a test method, both in terms of its bias and in terms of its precision. This practice
provides a standard procedure for determining the precision of a test method. Precision, when
evaluating test methods, is expressed in terms of two measurement concepts, repeatability and
reproducibility. Under repeatability conditions the factors listed above are kept or remain reasonably
constantandusuallycontributeonlyminimallytothevariability.Underreproducibilityconditionsthe
factors are generally different (that is, they change from laboratory to laboratory) and usually
contribute appreciably to the variability of test results.Thus, repeatability and reproducibility are two
practical extremes of precision.
The repeatability measure, by excluding the factors a through d as contributing variables, is not
intended as a mechanism for verifying the ability of a laboratory to maintain“ in-control” conditions
for routine operational factors such as operator-to-operator and equipment differences or any effects
of longer time intervals between test results. Such a control study is a separate issue for each
laboratory to consider for itself, and is not a recommended part of an interlaboratory study.
The reproducibility measure (including the factors a through d as sources of variability) reflects
whatprecisionmightbeexpectedwhenrandomportionsofahomogeneoussamplearesenttorandom
“in-control” laboratories.
To obtain reasonable estimates of repeatability and reproducibility precision, it is necessary in an
interlaboratory study to guard against excessively sanitized data in the sense that only the uniquely
best operators are involved or that a laboratory takes unusual steps to get “good” results. It is also
importanttorecognizeandconsiderhowtotreat“poor”resultsthatmayhaveunacceptableassignable
causes (for example, departures from the prescribed procedure). The inclusion of such results in the
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
1

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E691 − 14
final precision estimates might be questioned.
Anessentialaspectofcollectingusefulconsistentdataiscarefulplanningandconductofthestudy.
Questionsconcerningthenumberoflaboratoriesrequiredforasuccessfulstudyaswellasthenumber
of test results per labo
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E691 − 13 E691 − 14 An American National Standard
Standard Practice for
Conducting an Interlaboratory Study to Determine the
1
Precision of a Test Method
This standard is issued under the fixed designation E691; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript 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.
INTRODUCTION
Tests performed on presumably identical materials in presumably identical circumstances do not, in
general, yield identical results. This is attributed to unavoidable random errors inherent in every test
procedure; the factors that may influence the outcome of a test cannot all be completely controlled. In
the practical interpretation of test data, this inherent variability has to be taken into account. For
instance, the difference between a test result and some specified value may be within that which can
be expected due to unavoidable random errors, in which case a real deviation from the specified value
has not been demonstrated. Similarly, the difference between test results from two batches of material
will not indicate a fundamental quality difference if the difference is no more than can be attributed
to inherent variability in the test procedure. Many different factors (apart from random variations
between supposedly identical specimens) may contribute to the variability in application of a test
method, including: a the operator, b equipment used, c calibration of the equipment, and d
environment (temperature, humidity, air pollution, etc.). It is considered that changing laboratories
changes each of the above factors. The variability between test results obtained by different operators
or with different equipment will usually be greater than between test results obtained by a single
operator using the same equipment. The variability between test results taken over a long period of
time even by the same operator will usually be greater than that obtained over a short period of time
because of the greater possibility of changes in each of the above factors, especially the environment.
The general term for expressing the closeness of test results to the “true” value or the accepted
reference value is accuracy. To be of practical value, standard procedures are required for determining
the accuracy of a test method, both in terms of its bias and in terms of its precision. This practice
provides a standard procedure for determining the precision of a test method. Precision, when
evaluating test methods, is expressed in terms of two measurement concepts, repeatability and
reproducibility. Under repeatability conditions the factors listed above are kept or remain reasonably
constant and usually contribute only minimally to the variability. Under reproducibility conditions the
factors are generally different (that is, they change from laboratory to laboratory) and usually
contribute appreciably to the variability of test results. Thus, repeatability and reproducibility are two
practical extremes of precision.
The repeatability measure, by excluding the factors a through d as contributing variables, is not
intended as a mechanism for verifying the ability of a laboratory to maintain“ in-control” conditions
for routine operational factors such as operator-to-operator and equipment differences or any effects
of longer time intervals between test results. Such a control study is a separate issue for each
laboratory to consider for itself, and is not a recommended part of an interlaboratory study.
The reproducibility measure (including the factors a through d as sources of variability) reflects
what precision might be expected when random portions of a homogeneous sample are sent to random
“in-control” laboratories.
1
This practice is under the jurisdiction of ASTM Committee E11 on Quality and Statistics and is the direct responsibility of Subcommittee E11.20 on Test Method
Evaluation and Quality Control.
Current edition approved May 1, 2013April 1, 2014. Published May 2013May 2014. Originally approved in 1979. Last previous edition approved in 20122013 as
E691 – 12.E691 – 13. DOI: 10.1520/E0691-13.10.1520/E0691-14.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
1

---------------------- Page: 1 --
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5.5 When the conditions of adequate within-sample homogeneity and between-sample homogeneity are satisfied, any differences in test results on multiple samples can reasonably be attributed to testing variation and not due to sample variation.  
5.6 When differences within or between samples are discovered and the samples are deemed insufficiently homogeneous, the sample preparation process can be improved or corrected and a new set of samples can be prepared. Or, in cases where the sample homogeneity cannot be improved or for other reasons when the samples must be used, the method of eval...
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1.2 This guide is applicable to samples and reference materials used for proficiency testing programs and for interlaboratory studies to determine precision estimates for test methods. It may also be useful for activities related to quality control of testing within a single laboratory.  
1.3 Five techniques are presented for assessing sample homogeneity. The five techniques are not an exhaustive list of available techniques for assessing homogeneity of samples, but the techniques were chosen to cover a range of circumstances (and various degrees of rigor required) for laboratory studies of various types and purposes.  
1.4 Each of the first four techniques provides a scheme for testing for homogeneity and a statistical procedure for evaluating the results of the homogeneity testing. The circumstances are described for which each of the techniques is suited.  
1.5 For circumstances when homogeneity testing is not possible, the fifth technique provides guidance for producing homogeneous samples.  
1.6 The appendixes of this guide provide example spreadsheets for Techniques 1, 2, 3, and 4.  
1.7 This guide is not intended for evaluation of certified reference materials (CRMs) or materials used for calibration.  
1.8 Units—The system of units for this standard is not specified. Dimensional quantities in the standard are presented only as illustrations of calculation methods. The examples are not binding on products or test methods treated.  
1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to ...

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ASTM
ASTM E177-20(Practice)
Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods

ABSTRACT
The purpose of this practice is to present concepts necessary to the understanding of the terms “precision” and “bias” as used in quantitative test methods. This practice also describes methods of expressing precision and bias and, in a final section, gives examples of how statements on precision and bias may be written for ASTM test methods. A statement of precision allows potential users of a test method to assess in general terms the test method’s usefulness with respect to variability in proposed applications.
SIGNIFICANCE AND USE
4.1 Part A of the “Blue Book,” Form and Style for ASTM Standards, requires that all test methods include statements of precision and bias. This practice discusses these two concepts and provides guidance for their use in statements about test methods.  
4.2 Precision—A statement of precision allows potential users of a test method to assess in general terms the test method’s usefulness with respect to variability in proposed applications. A statement of precision is not intended to exhibit values that can be exactly duplicated in every user’s laboratory. Instead, the statement provides guidelines as to the magnitude of variability that can be expected between test results when the method is used in one, or in two or more, reasonably competent laboratories. For a discussion of precision, see 8.1.  
4.3 Bias—A statement of bias furnishes guidelines on the relationship between a set of typical test results produced by the test method under specific test conditions and a related set of accepted reference values (see 9.1).  
4.3.1 An alternative term for bias is trueness, which has a positive connotation, in that greater bias is associated with less favorable trueness. Trueness is the systematic component of accuracy.  
4.4 Accuracy—The term “accuracy,” used in earlier editions of Practice E177, embraces both precision and bias (see 9.3).
SCOPE
1.1 The purpose of this practice is to present concepts necessary to the understanding of the terms “precision” and “bias” as used in quantitative test methods. This practice also describes methods of expressing precision and bias and, in a final section, gives examples of how statements on precision and bias may be written for ASTM test methods.  
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.3 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

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ASTM
ASTM E2554-18e1(Practice)
Standard Practice for Estimating and Monitoring the Uncertainty of Test Results of a Test Method Using Control Chart Techniques

ABSTRACT
This practice describes techniques for a laboratory to estimate the uncertainty of a test result using data from test results on a control sample. This practice provides one method for a laboratory to estimate Measurement Uncertainty in accordance with Section A22.3 in Form and Style for ASTM Standards. This practice describes the use of control charts to evaluate the data obtained and presents a special type of control chart to monitor the estimate of uncertainty.
This practice provides one way for a laboratory to develop data-based Type A estimates of uncertainty as referred to in Section A22 in Form and Style for ASTM Standards.
SIGNIFICANCE AND USE
4.1 This practice provides one way for a laboratory to develop data-based Type A estimates of uncertainty as referred to in Section A22 in Form and Style for ASTM Standards.  
4.2 Laboratories accredited under ISO/IEC 17025 are required to present uncertainty estimates for their test results. This practice provides procedures that use test results to develop uncertainty estimates for an individual laboratory.  
4.3 Generally, these test results will be from a single sample of stable and homogeneous material known as a control or check sample.  
4.4 The true value of the characteristic(s) of the control sample being measured will ordinarily be unknown. However, this methodology may also be used if the control sample is a reference material, in which case the test method bias may also be estimated and incorporated into the uncertainty estimate. Many test methods do not have true reference materials available to provide traceable chains of uncertainty estimation.  
4.5 This practice also allows for ongoing monitoring of the laboratory uncertainty. As estimates of the level of uncertainty change, possibly as contributions to uncertainty are identified and minimized, revision to the laboratory uncertainty will be possible.
SCOPE
1.1 This practice describes techniques for a laboratory to estimate the uncertainty of a test result using data from test results on a control sample. This practice provides one method for a laboratory to estimate Measurement Uncertainty in accordance with Section A22.3 in Form and Style for ASTM Standards.  
1.2 Uncertainty as defined by this practice applies to the capabilities of a single laboratory. Any estimate of uncertainty determined through the use of this practice applies only to the individual laboratory for which the data are presented.  
1.3 The laboratory uses a well defined and established test method in determining a series of test results. The uncertainty estimated using this practice only applies when the same test method is followed. The uncertainty only applies for the material types represented by the control samples, and multiple control samples may be needed, especially if the method has different precision for different sample types or response levels.  
1.4 The uncertainty estimate determined by this practice represents the intermediate precision of test results. This estimate seeks to quantify the total variation expected within a single laboratory using a single established test method while incorporating as many known sources of variation as possible.  
1.5 This practice does not establish error estimates (error budget) attributed to individual factors that could influence uncertainty.  
1.6 This practice describes the use of control charts to evaluate the data obtained and presents a special type of control chart to monitor the estimate of uncertainty.  
1.7 The system of units for this standard is not specified. Dimensional quantities in the standard are presented only as illustrations of calculation methods. The examples are not binding on products or test methods treated.  
1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmenta...

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