ASTM E177-90a(1996)
(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. Note 1-The term "accuracy", used in earlier editions of Practice E177, 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: Section Terminology 3 Significance and Use 4 GENERAL CONCEPTS Test Method 5 Measurement Terminology 6 Observation 7 Test Determination 8 Test Result 9 SOURCES OF VARIABILITY Experimental Realization of a Test Method 10 Operator 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 Material 21 Variation of Precision and Bias with Sources of Variability 22 COMBINATIONS OF SOURCES OF VARIABILITY Repeatability and Laboratory Bias 23 Other Within-a-Single Laboratory Precisions 24 Reproducibility and Bias of the Test Method 25 Range of Materials 26 METHODS OF EXPRESSING PRECISION AND BIAS Indexes of Precision 27 Preferred 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 Bias 31 APPENDIX Alphabetical List of Descriptions of Terms from the Text XI
1.3 This standard does not purport to address the safety problems 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.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.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: E 177 – 90a (Reapproved 1996)
AMERICAN SOCIETY FOR TESTING AND MATERIALS
100 Barr Harbor Dr., West Conshohocken, PA 19428
Reprinted from the Annual Book of ASTM Standards. Copyright ASTM
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
Reproducibility and Bias of the Test Method 25
Range of Materials 26
1.1 The purpose of this practice is to present concepts
necessary to the understanding of the terms “precision” and
METHODS OF EXPRESSING PRECISION AND BIAS
“bias” as used in quantitative test methods. This practice also
Indexes of Precision 27
describes methods of expressing precision and bias and, in a
Preferred Indexes of Precision for ASTM Test Methods 28
final section, gives examples of how statements on precision Preferred Statements of Bias for ASTM Test Methods 29
Elements of a Statement of Precision and Bias 30
and bias may be written for ASTM test methods.
STATEMENTS OF PRECISION AND BIAS
NOTE 1—The term “accuracy”, used in earlier editions of Practice
E 177, embraces both precision and bias (see Section 20 and Note 4).
Examples of Statements of Precision and Bias 31
1.2 Informal descriptions of the concepts are introduced in
APPENDIX
the text as the concepts are developed, and appear in the
following sections:
Alphabetical List of Descriptions of Terms from the Text Appendix
X1
Section
Terminology 3
1.3 This standard does not purport to address all of the
Significance and Use 4
safety concerns, if any, associated with its use. It is the
GENERAL CONCEPTS
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
Test Method 5
bility of regulatory limitations prior to use.
Measurement Terminology 6
Observation 7
Test Determination 8
2. Referenced Documents
Test Result 9
2.1 ASTM Standards:
SOURCES OF VARIABILITY
E 178 Practice for Dealing with Outlying Observations
E 456 Terminology Relating to Quality and Statistics
Experimental Realization of a Test Method 10
Operator 11 E 691 Practice for Conducting an Interlaboratory Study to
Apparatus 12
Determine the Precision of a Test Method
Environment 13
E 1169 Guide for Conducting Ruggedness Tests
Sample 14
2.2 ANSI/ASQC Standard:
Time 15
A1-1978 Definitions, Symbols, Formulas and Tables for
STATISTICAL CONCEPTS
Control Charts
2.3 Other Documents:
Accepted Reference Value 16
Statistical Control 17
TAPPI Collaborative Reference Program, Reports
Precision 18
25 through 51, Aug. 1973 through Jan. 1978
Bias 19
Accuracy 20 ASQC Glossary and Tables for Statistical Quality Control
Variation of Precision and Bias with Material 21
Variation of Precision and Bias with Sources of Variability 22
3. Terminology
COMBINATIONS OF SOURCES OF VARIABILITY 3.1 The terminology defined in Terminology E 456 applies
in all areas affected by this practice, except where modified by
Repeatability and Laboratory Bias 23
this practice.
Other Within-a-Single Laboratory Precisions 24
1 2
This practice is under the jurisdiction of ASTM Committee E-11 on Quality and Annual Book of ASTM Standards, Vol 14.02.
Statistics and is the direct responsibility of Subcommittee E11.20on 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.
NOTICE:¬This¬standard¬has¬either¬been¬superceded¬and¬replaced¬by¬a¬new¬version¬or¬discontinued.¬
Contact¬ASTM¬International¬(www.astm.org)¬for¬the¬latest¬information.¬
E 177
temperature would be introduced to establish the limits of significant
3.2 This practice is specifically concerned with the devel-
effect, (1, 2, 3).
opment of statements on precision and bias for inclusion as
descriptors of the performance of a test method. This applica- 5.4 A well-written test method specifies control over such
tion requires refinement of the Terminology E 456 definitions,
factors as the test equipment, the test environment, the quali-
as discussed herein. fications of the operator (explicitly or implicitly), the prepara-
tion of test specimens, and the operating procedure for using
3.3 The informal descriptions of concepts developed in this
the equipment in the test environment to measure some
practice have been collected in Appendix X1, and have been
property of the test specimens. The test method will also
arranged alphabetically for easy reference.
specify the number of test specimens required and how
measurements on them are to be combined to provide a test
4. Significance and Use
result (Section 9), and might also reference a sampling proce-
4.1 Part A of the “Blue Book,” Form and Style for ASTM
dure appropriate for the intended use of the method.
Standards, requires that all test methods include statements of
5.5 It is necessary that the writers of the test method provide
precision and bias. This practice discusses these two concepts
instructions or requirements for every known outside influence.
and provides guidance for their use in statements about test
methods.
6. Measurement Terminology
4.2 Precision—A statement of precision allows potential
6.1 The following terms have been used to describe both the
users of a test method to assess in general terms the test
measurement process and the partial or complete result of the
method’s usefulness with respect to variability in proposed
process: measurement, observation, observed value, test, test
applications. A statement on precision is not intended to
determination, test result, and others. These terms have often
contain values that can be exactly duplicated in every user’s
been used loosely and interchangeably.
laboratory. Instead, the statement provides guidelines as to the
6.2 For clarity, it is necessary to select certain of these terms
kind of variability that can be expected between test results
for specific use. However, the word “measurement” will be
when the method is used in one or more reasonably competent
used in a generic sense to cover observation (or observed
laboratories. For a discussion of precision, see Section 18.
value), test determination and test result. The use of the word
4.3 Bias—A statement on bias furnishes guidelines on the “test” by itself is discouraged.
6.3 A quantitative test method may have three distinct
relationship between a set of typical test results produced by
stages: (1) the direct measurement or observation of dimen-
the test method under specific test conditions and a related set
sions or properties; (2) the arithmetical combination of the
of accepted reference values (see Section 19).
observed values to obtain a single determination; and (3) the
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” is used to mean material, product, system or service;
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
value obtained from the process. Thus, the test determination
affect the test results (see Guide E 1169).
may summarize or combine one or more observations.
8.2 Examples:
NOTE 2—A screening procedure or ruggedness test is a procedure for
8.2.1 The measurement of the density of a test specimen
investigation of the effects of variations in environmental and other
may involve the separate observation of the mass and the
pertinent factors on the test results obtained from a test in order to
determine how control of such factors should be specified in the written
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
The boldface numbers in parentheses refer to a list of references at the end of
cases and less in others. In a screening procedure, deliberate variations in this standard.
NOTICE:¬This¬standard¬has¬either¬been¬superceded¬and¬replaced¬by¬a¬new¬version¬or¬discontinued.¬
Contact¬ASTM¬International¬(www.astm.org)¬for¬the¬latest¬information.¬
E 177
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
10.1 A realization of a test method refers to an actual
9.1 A test result is the value obtained by carrying out the
application of the test method to produce a test result as
complete protocol of the test method once, being either a single
specified by the test method. The realization involves an
test determination or a specified combination of a number of
interpretation of the written document by a specific test
test determinations.
operator, who uses a specific unit and version of the specified
9.2 In general, a test method describes not only the manner
test apparatus,inthe particular environment of his testing
in which each test determination is to be made, but also the
laboratory, to evaluate a specified number of test specimens of
number of test determinations to be made and how these are to
the material to be tested. Another realization of the test method
be combined to provide the test result.
may involve a change in one or more of the above emphasized
9.3 Examples:
experimental factors. The test result obtained by another
9.3.1 The test method on density might require that the mass
realization of the test method will usually differ from the test
and volume observations of a specimen be combined to give a
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.
changes usually occur. The outcome of these changes may be
9.3.2 The test method for paper thickness may require that
seen as variability among the test results.
the determination of strip thickness in 8.2.2 be made on ten
10.2 Each of the above experimental factors and all others,
strips and that the ten test determinations be averaged to give
known and unknown, that can change the realization of a test
the test result.
method, are potential sources of variability in test results. Some
9.3.3 The test method for a tensile strength test of paper may
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.
11.1 Clarity of Test Method—Every effort must be made in
9.3.4 In chemical analyses a variety of situations may occur.
preparing an ASTM standard test method to eliminate the
Thus, in some cases, the method may call for the preparation of
possibility of serious differences in interpretation. One way to
a single solution from a test unit, and measurement on three
check clarity is to observe, without comment, a competent
aliquots (specimens) of the solution made up to a specified
laboratory technician, not previously familiar with the method,
volume. The average of the three analytical determinations
apply the draft test method. If the technician has any difficulty,
would then be called the test result. In other cases of chemical
the draft most likely needs revision.
analysis, the method may call for two individual test determi-
11.2 Completeness of Test Method—It is necessary that
nations, each one made on a different specimen with recalibra-
technicians, who are generally familiar with the test method or
tion of the measuring instrument for each of the two determi-
similar methods, not read anything into the instructions that is
nations. The average of the two determinations would then be
not explicitly stated therein. Therefore, to ensure minimum
the test result.
variability due to interpretation, procedural requirements must
9.3.5 In rubber testing, the method may describe not only
be complete.
the
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.