ASTM D6708-06
(Practice)Standard Practice for Statistical Assessment and Improvement of Expected Agreement Between Two Test Methods that Purport to Measure the Same Property of a Material
Standard Practice for Statistical Assessment and Improvement of Expected Agreement Between Two Test Methods that Purport to Measure the Same Property of a Material
SCOPE
1.1 This practice covers statistical methodology for assessing the expected agreement between two standard test methods that purport to measure the same property of a material, and deciding if a simple linear bias correction can further improve the expected agreement. It is intended for use with results collected from an interlaboratory study meeting the requirement of Practice D 6300 or equivalent (for example, ISO 4259). The interlaboratory study must be conducted on at least ten materials that span the intersecting scopes of the test methods, and results must be obtained from at least six laboratories using each method.Note 1
Examples of standard test methods are those developed by voluntary consensus standards bodies such as ASTM, IP/BSI, DIN, AFNOR, CGSB.
1.2 The statistical methodology is based on the premise that a bias correction will not be needed. In the absence of strong statistical evidence that a bias correction would result in better agreement between the two methods, a bias correction is not made. If a bias correction is required, then the parsimony principle is followed whereby a simple correction is to be favored over a more complex one.Note 2
Failure to adhere to the parsimony principle generally results in models that are over-fitted and do not perform well in practice.
1.3 The bias corrections of this practice are limited to a constant correction, proportional correction or a linear (proportional + constant) correction.
1.4 The bias-correction methods of this practice are method symmetric, in the sense that equivalent corrections are obtained regardless of which method is bias-corrected to match the other.
1.5 A methodology is presented for establishing the 95 % confidence limit (designated by this practice as the cross-method reproducibility) for the difference between two results where each result is obtained by a different operator using different apparatus and each applying one of the two methods X and Y on identical material, where one of the methods has been appropriately bias-corrected in accordance with this practice.Note 3
Users are cautioned against applying the cross-method reproducibility as calculated from this practice to materials that are significantly different in composition from those actually studied, as the ability of this practice to detect and address sample-specific biases (see ) is dependent on the materials selected for the interlaboratory study. When sample-specific biases are present, the types and ranges of samples may need to be expanded significantly from the minimum of ten as specified in this practice in order to obtain a more comprehensive and reliable 95 % confidence limits for cross method reproducibility that adequately cover the range of sample specific biases for different types of materials.
1.6 This practice is intended for test methods which measure quantitative (numerical) properties of petroleum or petroleum products.
1.7 The statistical methodology outlined in this practice is also applicable for assessing the expected agreement between any two test methods that purport to measure the same property of a material, provided the results are obtained on the same comparison sample set, the standard error associated with each test result is known, the sample set design meets the requirement of this practice, and the statistical degree of freedom of the data set exceeds 30.
1.8 Software program CompTM Version 1.0.21 (ADJD6708) performs the necessary computations prescribed by this practice.
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An American National Standard
Designation: D 6708 – 06
Standard Practice for
Statistical Assessment and Improvement of Expected
Agreement Between Two Test Methods that Purport to
1
Measure the Same Property of a Material
This standard is issued under the fixed designation D6708; 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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope different apparatus and each applying one of the two methods
X and Y on identical material, where one of the methods has
1.1 This practice covers statistical methodology for assess-
been appropriately bias-corrected in accordance with this
ingtheexpectedagreementbetweentwostandardtestmethods
practice.
that purport to measure the same property of a material, and
deciding if a simple linear bias correction can further improve
NOTE 3—Users are cautioned against applying the cross-method repro-
the expected agreement. It is intended for use with results
ducibilityascalculatedfromthispracticetomaterialsthataresignificantly
different in composition from those actually studied, as the ability of this
collected from an interlaboratory study meeting the require-
practicetodetectandaddresssample-specificbiases(see6.8)isdependent
ment of Practice D 6300 or equivalent (for example,
on the materials selected for the interlaboratory study. When sample-
ISO4259). The interlaboratory study must be conducted on at
specific biases are present, the types and ranges of samples may need to
least ten materials that span the intersecting scopes of the test
be expanded significantly from the minimum of ten as specified in this
methods, and results must be obtained from at least six
practice in order to obtain a more comprehensive and reliable 95%
laboratories using each method.
confidence limits for cross method reproducibility that adequately cover
the range of sample specific biases for different types of materials.
NOTE 1—Examples of standard test methods are those developed by
voluntary consensus standards bodies such as ASTM, IP/BSI, DIN, 1.6 This practice is intended for test methods which mea-
AFNOR, CGSB.
sure quantitative (numerical) properties of petroleum or petro-
leum products.
1.2 Thestatisticalmethodologyisbasedonthepremisethat
1.7 The statistical methodology outlined in this practice is
a bias correction will not be needed. In the absence of strong
also applicable for assessing the expected agreement between
statistical evidence that a bias correction would result in better
anytwotestmethodsthatpurporttomeasurethesameproperty
agreement between the two methods, a bias correction is not
of a material, provided the results are obtained on the same
made. If a bias correction is required, then the parsimony
comparison sample set, the standard error associated with each
principle is followed whereby a simple correction is to be
test result is known, the sample set design meets the require-
favored over a more complex one.
ment of this practice, and the statistical degree of freedom of
NOTE 2—Failure to adhere to the parsimony principle generally results
the data set exceeds 30.
in models that are over-fitted and do not perform well in practice.
1.8 Software program CompTM Version 1.0.21
1.3 The bias corrections of this practice are limited to a
(ADJD6708) performs the necessary computations prescribed
constantcorrection,proportionalcorrectionoralinear(propor-
by this practice.
tional + constant) correction.
2. Referenced Documents
1.4 The bias-correction methods of this practice are method
2
symmetric,inthesensethatequivalentcorrectionsareobtained
2.1 ASTM Standards:
regardless of which method is bias-corrected to match the
D5580 Test Method for Determination of Benzene, Tolu-
other.
ene, Ethylbenzene, p/m-Xylene, o-Xylene, C and Heavier
9
1.5 A methodology is presented for establishing the 95%
Aromatics, and Total Aromatics in Finished Gasoline by
confidence limit (designated by this practice as the cross-
Gas Chromatography
method reproducibility) for the difference between two results
D5769 Test Method for Determination of Benzene, Tolu-
where each result is obtained by a different operator using
ene, and Total Aromatics in Finished Gasolines by Gas
1
This practice is under the jurisdiction ofASTM Committee D02 on Petroleum
2
ProductsandLubricantsandisthedirectresponsibilityofSubcommitteeD02.94on For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Quality Assurance and Statistics. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
CurrenteditionapprovedJuly1,2006.PublishedJuly2006.Originallyapproved Standards volume information, refer to the stand
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