ASTM D6708-21

1.1''This practice covers statistical methodology for assessing the expected agreement between two different standard test methods that purport to measure the same property of a material, and for the purpose of deciding if a simple linear bias correction can further improve the expected agreement. It is intended for use with results obtained from interlaboratory studies meeting the requirement of Practice D6300 or equivalent (for example, ISO'''4259). The interlaboratory studies shall be conducted on at least ten materials in common that among them span the intersecting scopes of the test methods, and results shall be obtained from at least six laboratories using each method. Requirements in this practice shall be met in order for the assessment to be considered suitable for publication in either method, if such publication includes claim to have been carried out in compliance with this practice. Any such publication shall include mandatory information regarding certain details of the assessment outcome as specified in the Report section of this practice.

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.

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 numerical limit (designated by this practice as the between methods reproducibility) that would be exceeded about 5'''% of the time (one case in 20 in the long run) 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, in the normal and correct operation of both test methods.

1.6''This practice is intended for test methods which measure quantitative (numerical) properties of petroleum or petroleum products.

1.7''The statistical calculations of this practice are also applicable for assessing the expected agreement between two different test methods that purport to measure the same property of a material using results that are not as described in 1.1, provided the results and associated statistics from each test method are obtained from a specifically designed multi-lab study or from a proficiency testing program (e.g.: ILCP) where for each sample a single result is provided by each lab for each test method. The comparison sample set shall comprise at least ten different materials that span the intersecting scopes of the test methods with no material exceeding the leverage requirement in Practice D6300. Results and statistics shall meet requirements in 1.7.1. Requirements in this practice shall be met in order for the assessment to be considered suitable for publication in either method, if such publication includes claim to have been carried out in compliance with this practice. Any such publication shall include mandatory information regarding certain details of the assessment as specified in the Report section of this practice. R_XY shall be based on the published reproducibility of the methods.