ASTM_D_6708_-_15.pdf

Designation:D670815An American National StandardStandard Practice forStatistical Assessment and Improvement of ExpectedAgreement Between Two Test Methods that Purport toMeasure the Same Property of a Material1This standard is issued under the fixed designation D6708;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon()indicates an editorial change since the last revision or reapproval.1.Scope*1.1 This practice covers statistical methodology for assess-ing the expected agreement between two standard test methodsthat purport to measure the same property of a material,anddeciding if a simple linear bias correction can further improvethe expected agreement.It is intended for use with resultscollected from an interlaboratory study meeting the require-ment of Practice D6300 or equivalent(for example,ISO 4259).The interlaboratory study must be conducted on at least tenmaterials that span the intersecting scopes of the test methods,and results must be obtained from at least six laboratories usingeach method.1.2 The statistical methodology is based on the premise thata bias correction will not be needed.In the absence of strongstatistical evidence that a bias correction would result in betteragreement between the two methods,a bias correction is notmade.If a bias correction is required,then the parsimonyprinciple is followed whereby a simple correction is to befavored over a more complex one.NOTE1Failure to adhere to the parsimony principle generally resultsin models that are over-fitted and do not perform well in practice.1.3 The bias corrections of this practice are limited to aconstant correction,proportional correction or a linear(propor-tional+constant)correction.1.4 The bias-correction methods of this practice are methodsymmetric,in the sense that equivalent corrections are obtainedregardless of which method is bias-corrected to match theother.1.5 A methodology is presented for establishing the 95%confidence limit(designated by this practice as the betweenmethods reproducibility)for the difference between two resultswhere each result is obtained by a different operator usingdifferent apparatus and each applying one of the two methodsX and Y on identical material,where one of the methods hasbeen appropriately bias-corrected in accordance with thispractice.NOTE2In earlier versions of this standard practice,the term“cross-method reproducibility”was used in place of the term“between methodsreproducibility.”The change was made because the“between methodsreproducibility”term is more intuitive and less confusing.It is importantto note that these two terms are synonymous and interchangeable with oneanother,especially in cases where the“cross-method reproducibility”termwas subsequently referenced by name in methods where a D6708assessment was performed,before the change in terminology in thisstandard practice was adopted.NOTE3Users are cautioned against applying the between methodsreproducibility as calculated from this practice to materials that aresignificantly different in composition from those actually studied,as theability of this practice to detect and address sample-specific biases(see6.8)is dependent on the materials selected for the interlaboratory study.When sample-specific biases are present,the types and ranges of samplesmay need to be expanded significantly from the minimum of ten asspecified in this practice in order to obtain a more comprehensive andreliable 95%confidence limits for between methods reproducibility thatadequately cover the range of sample specific biases for different types ofmaterials.1.6 This practice is intended for test methods which mea-sure quantitative(numerical)properties of petroleum or petro-leum products.1.7 The statistical methodology outlined in this practice isalso applicable for assessing the expected agreement betweenany two test methods that purport to measure the same propertyof a material,provided the results are obtained on the samecomparison sample set,the standard error associated with eachtest result is known,the sample set design meets the require-ment of this practice,and the statistical degree of freedom ofthe data set exceeds 30.2.Referenced Documents2.1 ASTM Standards:2D5580 Test Method for Determination of Benzene,Toluene,Ethylbenzene,p/m-Xylene,o-Xylene,C9and Heavier1This practice is under the jurisdiction of ASTM Committee D02 on PetroleumProducts,Liquid Fuels,and Lubricants and is the direct responsibility of Subcom-mittee D02.94 on Coordinating Subcommittee on Quality Assurance and Statistics.Current edition approved July 1,2015.Published August 2015.Originallyapproved in 2001.Last previous edition approved in 2013 as D6708 131.DOI:10.1520/D6708-15.2For referenced ASTM standards,visit the ASTM website,www.astm.org,orcontact ASTM Customer Service at serviceastm.org.For Annual Book of AST