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.
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 and health practices and determine the applicability of regulatory requirements prior to use.

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Status
Historical
Publication Date
31-Oct-2004
Current Stage
Ref Project

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Effective Date
01-Nov-2004

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ASTM E177-04e1 - Standard Practice for Use of the Terms Precision and Bias in ASTM Test Methods
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An American National Standard
e1
Designation:E177–04
Standard Practice for
1
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
e NOTE—Editorial corrections were made in 27.3.3 and 28.2 in April 2005.
1. Scope (3) a consensus or certified value, based on collaborative
experimental work under the auspices of a scientific or
1.1 The purpose of this practice is to present concepts
engineering group.
necessary to the understanding of the terms “precision” and
3.1.2 accuracy, n—the closeness of agreement between a
“bias” as used in quantitative test methods. This practice also
test result and an accepted reference value.
describes methods of expressing precision and bias and, in a
3.1.2.1 Discussion—The term accuracy, when applied to a
final section, gives examples of how statements on precision
set of test results, involves a combination of a random
and bias may be written for ASTM test methods.
component and of a common systematic error or bias compo-
1.2 This standard does not purport to address all of the
nent.
safety concerns, if any, associated with its use. It is the
3.1.3 bias, n—the difference between the expectation of the
responsibility of the user of this standard to establish appro-
test results and an accepted reference value.
priate safety and health practices and determine the applica-
3.1.3.1 Discussion—Bias is the total systematic error as
bility of regulatory requirements prior to use.
contrasted to random error. There may be one or more
2. Referenced Documents systematic error components contributing to the bias. A larger
2
systematic difference from the accepted reference value is
2.1 ASTM Standards:
reflected by a larger bias value.
E 178 Practice for Dealing with Outlying Observations
3.1.4 intermediate precision, n—the closeness of agreement
E 456 Terminology Relating to Quality and Statistics
between test results obtained under specified intermediate
E 691 Practice for Conducting an Interlaboratory Study to
precision conditions.
Determine the Precision of a Test Method
3.1.4.1 Discussion—The specific measure and the specific
E 1169 Guide for Conducting Ruggedness Tests
conditions must be specified for each intermediate measure of
2.2 Other Document:
3
precision; thus, “standard deviation of test results among
ISO 3534 Statistics—Vocabulary and Symbols
operators in a laboratory,” or “day-to-day standard deviation
3. Terminology
within a laboratory for the same operator.”
3.1.4.2 Discussion—Because the training of operators, the
3.1 Definitions:
agreement of different pieces of equipment in the same
3.1.1 accepted reference value, n—a value that serves as an
laboratory and the variation of environmental conditions with
agreed-upon reference for comparison, and which is derived
longer time intervals all depend on the degree of within-
as: (1) a theoretical or established value, based on scientific
laboratory control, the intermediate measures of precision are
principles, (2) an assigned or certified value, based on experi-
likely to vary appreciably from laboratory to laboratory. Thus,
mental work of some national or international organization, or
intermediate precisions may be more characteristic of indi-
vidual laboratories than of the test method.
1
This practice is under the jurisdiction ofASTM Committee E11 on Quality and
3.1.5 intermediate precision conditions, n—conditions un-
Statistics and is the direct responsibility of Subcommittee E11.20 on Test Method
der which test results are obtained with the same test method
Evaluation and Quality Control.
using test units or test specimens (10.3) taken at random from
Current edition approved Nov. 1, 2004. Published December 2004. Originally
approved in 1961. Last previous edition approved in 2002 as E 177 – 90a (2002). a single quantity of material that is as nearly homogeneous as
2
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
possible, and with changing conditions such as operator,
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
measuring equipment, location within the laboratory, and time.
Standards volume information, refer to the standard’s Document Summary page on
3.1.6 precision, n—the closeness of agreement between
the ASTM website.
3
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