ASTM E177-90a(2002)
(Practice)Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
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. The term "accuracy", used in earlier editions of Practice E 177, embraces both precision and bias (see Section 20 and Note 4).
1.2 Informal descriptions of the concepts are introduced in the text as the concepts are developed, and appear in the following sections:SectionTerminology3 Significance and Use4
GENERAL CONCEPTS
Test Method5 Measurement Terminology6Observation7 Test Determination8Test Result9
SOURCES OF VARIABILITY
Experimental Realization of a Test Method10Operator 11 Apparatus 12 Environment 13 Sample 14 Time 15
STATISTICAL CONCEPTS
Accepted Reference Value 16 Statistical Control 17 Precision 18 Bias 19 Accuracy 20 Variation of Precision and Bias with Material21 Variation of Precision and Bias with Sources of Variability22
COMBINATIONS OF SOURCES OF VARIABILITY
Repeatability and Laboratory Bias 23 Other Within-a-Single Laboratory Precisions24 Reproducibility and Bias of the Test Method25 Range of Materials 26
METHODS OF EXPRESSING PRECISION AND BIAS
Indexes of Precision 27Preferred Indexes of Precision for ASTM Test Methods 28 Preferred Statements of Bias for ASTM Test Methods 29 Elements of a Statement of Precision and Bias 30
STATEMENTS OF PRECISION AND BIAS
Examples of Statements of Precision and Bias31
APPENDIXAlphabetical List of Descriptions of Terms from the Text X1
1.3 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 and health practices and determine the applicability of regulatory limitations prior to use.
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An American National Standard
Designation:E177–90a (Reapproved 2002)
Standard Practice for
Use of the Terms Precision and Bias in ASTM Test Methods
This standard is issued under the fixed designation E 177; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope
COMBINATIONS OF SOURCES OF VARIABILITY
1.1 The purpose of this practice is to present concepts
Repeatability and Laboratory Bias 23
necessary to the understanding of the terms “precision” and
Other Within-a-Single Laboratory Precisions 24
Reproducibility and Bias of the Test Method 25
“bias” as used in quantitative test methods. This practice also
Range of Materials 26
describes methods of expressing precision and bias and, in a
final section, gives examples of how statements on precision
METHODS OF EXPRESSING PRECISION AND BIAS
and bias may be written for ASTM test methods.
Indexes of Precision 27
Preferred Indexes of Precision for ASTM Test Methods 28
NOTE 1—The term “accuracy”, used in earlier editions of Practice
Preferred Statements of Bias for ASTM Test Methods 29
E 177, embraces both precision and bias (see Section 20 and Note 4).
Elements of a Statement of Precision and Bias 30
1.2 Informal descriptions of the concepts are introduced in
STATEMENTS OF PRECISION AND BIAS
the text as the concepts are developed, and appear in the
following sections:
Examples of Statements of Precision and Bias 31
Section
APPENDIX
Terminology 3
Significance and Use 4
Alphabetical List of Descriptions of Terms from the Text Appendix
X1
GENERAL CONCEPTS
1.3 This standard does not purport to address all of the
Test Method 5
safety concerns, if any, associated with its use. It is the
Measurement Terminology 6
Observation 7
responsibility of the user of this standard to establish appro-
Test Determination 8
priate safety and health practices and determine the applica-
Test Result 9
bility of regulatory limitations prior to use.
SOURCES OF VARIABILITY
2. Referenced Documents
Experimental Realization of a Test Method 10
2.1 ASTM Standards:
Operator 11
Apparatus 12
E 178 Practice for Dealing with Outlying Observations
Environment 13
E 456 Terminology Relating to Quality and Statistics
Sample 14
Time 15 E 691 Practice for Conducting an Interlaboratory Study to
Determine the Precision of a Test Method
STATISTICAL CONCEPTS
E 1169 Guide for Conducting Ruggedness Tests
2.2 ANSI/ASQC Standard:
Accepted Reference Value 16
Statistical Control 17
A1-1978 Definitions, Symbols, Formulas and Tables for
Precision 18
Control Charts
Bias 19
Accuracy 20 2.3 Other Documents:
Variation of Precision and Bias with Material 21
TAPPI Collaborative Reference Program, Reports
Variation of Precision and Bias with Sources of Variability 22
25 through 51, Aug. 1973 through Jan. 1978
1 2
This practice is under the jurisdiction ofASTM Committee E11 on Quality and Annual Book of ASTM Standards, Vol 14.02.
Statistics and is the direct responsibility of Subcommittee E11.20 on Test Method Available from American Society for Quality Control, 230 West Wells St.,
Evaluation and Quality Control. Milwaukee, WI 53203.
Current edition approved June 29, 1990. Published August 1990. Originally Available from the Technical Association of the Pulp and Paper Industry,
published as E 177 – 61. Last previous edition E 177 – 90. Technology Park/Atlanta, P.O. Box 105113, Atlanta, GA 30348.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E177–90a (2002)
determine how control of such factors should be specified in the written
ASQC Glossary and Tables for Statistical Quality Control
description of the method. For example, temperature of the laboratory or
of a heating device used in the test may have a significant effect in some
3. Terminology
cases and less in others. In a screening procedure, deliberate variations in
3.1 The terminology defined in Terminology E 456 applies
temperature would be introduced to establish the limits of significant
in all areas affected by this practice, except where modified by 5
effect, (1, 2, 3).
this practice.
5.4 A well-written test method specifies control over such
3.2 This practice is specifically concerned with the devel-
factors as the test equipment, the test environment, the quali-
opment of statements on precision and bias for inclusion as
fications of the operator (explicitly or implicitly), the prepara-
descriptors of the performance of a test method. This applica-
tion of test specimens, and the operating procedure for using
tion requires refinement of the Terminology E 456 definitions,
the equipment in the test environment to measure some
as discussed herein.
property of the test specimens. The test method will also
3.3 The informal descriptions of concepts developed in this
specify the number of test specimens required and how
practice have been collected in Appendix X1, and have been
measurements on them are to be combined to provide a test
arranged alphabetically for easy reference.
result (Section 9), and might also reference a sampling proce-
dure appropriate for the intended use of the method.
4. Significance and Use
5.5 Itisnecessarythatthewritersofthetestmethodprovide
4.1 Part A of the “Blue Book,” Form and Style for ASTM
instructionsorrequirementsforeveryknownoutsideinfluence.
Standards, requires that all test methods include statements of
precision and bias. This practice discusses these two concepts
6. Measurement Terminology
and provides guidance for their use in statements about test
6.1 Thefollowingtermshavebeenusedtodescribeboththe
methods.
measurement process and the partial or complete result of the
4.2 Precision—A statement of precision allows potential
process: measurement, observation, observed value, test, test
users of a test method to assess in general terms the test
determination, test result, and others. These terms have often
method’s usefulness with respect to variability in proposed
been used loosely and interchangeably.
applications. A statement on precision is not intended to
6.2 Forclarity,itisnecessarytoselectcertainoftheseterms
contain values that can be exactly duplicated in every user’s
for specific use. However, the word “measurement” will be
laboratory. Instead, the statement provides guidelines as to the
used in a generic sense to cover observation (or observed
kind of variability that can be expected between test results
value), test determination and test result. The use of the word
when the method is used in one or more reasonably competent
“test” by itself is discouraged.
laboratories. For a discussion of precision, see Section 18.
6.3 A quantitative test method may have three distinct
4.3 Bias—A statement on bias furnishes guidelines on the
stages: (1) the direct measurement or observation of dimen-
relationship between a set of typical test results produced by
sions or properties; (2) the arithmetical combination of the
the test method under specific test conditions and a related set
observed values to obtain a single determination; and (3) the
of accepted reference values (see Section 19).
arithmetical combination of a number of determinations to
GENERAL CONCEPTS
obtain the test result of the test method. These three stages are
explained and illustrated in Sections 7-9.
5. Test Method
5.1 Section 2 of the ASTM Regulations describes a test
7. Observation
method as “a definitive procedure for the identification, mea-
7.1 For the purposes of this practice, observation or ob-
surement, and evaluation of one or more qualities, character-
served value should be interpreted as the most elemental single
istics, or properties of a material, product, system or service
reading or corrected reading obtained in the process of making
that produces a test result.”
a measurement. This statement is a narrower interpretation
5.2 In this practice only quantitative test methods that
than is given in Terminology E 456 in that the latter applies to
produce numerical results are considered. Also, the word
nonquantitative as well as quantitative test methods.
“material”isusedtomeanmaterial,product,systemorservice;
7.2 An observation may involve a direct reading (for ex-
the word “property” is used herein to mean that a quantitative
ample, a zero-adjusted micrometer reading of the thickness of
test result can be obtained that describes a characteristic or a
a test strip at one position along the strip) or it may require the
quality, or some other aspect of the material; and “test method”
interpolation of the reading from a calibration curve.
refers to both the document and the procedure described
therein for obtaining a quantitative test result for one property.
8. Test Determination
For a discussion of test result, see Section 9.
8.1 For a quantitative test method, a test determination may
5.3 During its development, a test method should be sub-
be described as (1) the process of calculating from one or more
jected to a screening procedure and ruggedness test in order to
observations a property of a single test specimen, or as (2) the
establish the proper degree of control over factors that may
affect the test results (see Guide E 1169).
NOTE 2—A screening procedure or ruggedness test is a procedure for
investigation of the effects of variations in environmental and other
The boldface numbers in parentheses refer to a list of references at the end of
pertinent factors on the test results obtained from a test in order to this standard.
E177–90a (2002)
value obtained from the process. Thus, the test determination beingconvertedintoatestresult.Suchtreatmentmightinclude
may summarize or combine one or more observations. separation of the analytical response for the substance of
8.2 Examples: interest from the chromatographic absorption data, elimination
8.2.1 The measurement of the density of a test specimen or other treatment of outliers (see Practice E 178) in the data
may involve the separate observation of the mass and the for the known standard substances, and preparation of a
volume of the specimen and the calculation of the ratio calibration curve to determine the test result.
mass/volume. The density calculated from the ratio of one pair 9.4 Precision statements for ASTM test methods are appli-
of mass and volume observations made on one specimen is a cable to comparisons between test results, not test determina-
test determination. tions nor observations, unless specifically and clearly indicated
8.2.2 The determination of the thickness of a test specimen otherwise (see Section 18).
strip may involve averaging micrometer caliper observations
SOURCES OF VARIABILITY
taken at several points along the strip.
10. Experimental Realization of a Test Method
9. Test Result
9.1 A test result is the value obtained by carrying out the 10.1 A realization of a test method refers to an actual
application of the test method to produce a test result as
completeprotocolofthetestmethodonce,beingeitherasingle
test determination or a specified combination of a number of specified by the test method. The realization involves an
interpretation of the written document by a specific test
test determinations.
9.2 In general, a test method describes not only the manner operator, who uses a specific unit and version of the specified
test apparatus,inthe particular environment of his testing
in which each test determination is to be made, but also the
number of test determinations to be made and how these are to laboratory, to evaluate a specified number of test specimens of
the material to be tested.Another realization of the test method
be combined to provide the test result.
9.3 Examples: may involve a change in one or more of the above emphasized
experimental factors. The test result obtained by another
9.3.1 Thetestmethodondensitymightrequirethatthemass
and volume observations of a specimen be combined to give a realization of the test method will usually differ from the test
result obtained from the first realization. Even when none of
test determination of density (8.2.1) and the test determination
the experimental factors is intentionally changed, small
of each of five specimens be averaged to give a test result.
9.3.2 The test method for paper thickness may require that changes usually occur. The outcome of these changes may be
seen as variability among the test results.
the determination of strip thickness in 8.2.2 be made on ten
strips and that the ten test determinations be averaged to give 10.2 Each of the above experimental factors and all others,
known and unknown, that can change the realization of a test
the test result.
9.3.3 Thetestmethodforatensilestrengthtestofpapermay method,arepotentialsourcesofvariabilityintestresults.Some
of the more common factors are discussed in Sections 11-15.
specify that a tensile strength determination be performed on
each of ten specimens and that the ten tensile test determina-
11. Operator
tions be averaged to get the test result.
9.3.4 Inchemicalanalysesavarietyofsituationsmayoccur. 11.1 Clarity of Test Method—Every effort must be made in
Thus,insomecases,themethodmaycallforthepreparationof preparing an ASTM standard test method to eliminate the
a single solution from a test unit, and measurement on three possibility of serious differences in interpretation. One way to
aliquots (specimens) of the solution made up to a specified check clarity is to observe, without comment, a competent
volume. The average of the three analytical determinations laboratory technician, not previously familiar with the method,
would then be called the test result. In other cases of chemical apply the draft test method. If the technician has any difficulty,
analysis, the method may call for two individual test determi- the draft most likely needs revision.
11.2 Completeness of Test Method—It is necessary that
nations, each one made on a different specimen with recalibra-
tion of the measuring instrument for each of the two determi- technicians, who are generally familiar with the test method or
similar methods, not read anything into the instructions that is
nations. The average of the two determinations would then be
the test result. not explicitly stated therein. Therefore, to ensure minimum
9.3.5 In rubber testing, the method may describe not only variability due to interpretation, procedural requirements must
the shape of the test specimen to be taken from a sheet of be complete.
rubber, but also the preparation of the sheet, including com- 11.2.1 If requirement
...
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