11. Precision and Bias
11.1 The precision and bias of the measurements obtained with this test method will depend upon strict adherence to the stated test parameters and maintenance of the proper sand flow rate and sand curtain throughout the duration of the test.
11.2 The degree of agreement in repeated tests on the same material will depend upon material homogeneity, machine and material interaction, and close observation of the test by a competent machine operator.
11.3 Normal variations in the abrasive, rubber wheel char- acteristics, and procedure will tend to reduce the accuracy of this test method as compared to the accuracy of such material property tests as hardness or density.
11.4 Initial Machine Operation and Qualification The number of tests to establish precision and bias of the apparatus for initial machine operation shall be at least five. After initial qualification, a minimum of three tests may be used to periodically monitor precision and bias. These tests shall be made using Reference Materials (11.6) and the statistical caculations made, using formulas described in Practice E 122.
11.4.1 Tables X1.1 and X1.2 show representative coeffi- cients of variation or standard deviations, or both, which were obtained in the interlaboratory tests. The coefficient of varia- tion or the standard deviation, or both, for reference materials shall not exceed the values reported. If this value is exceeded, the machine operation shall be considered out of control and steps taken to eliminate erratic results.
11.4.2 The coefficient of variation tends to be higher for materials with volume losses in the range from 1 to 5 mm3. In such cases, the coefficient of variation is about 20 %.
11.4.3 In any test series all data must be considered in the calculation including outliers (data exceeding the obvious range). For example, an exceedingly high- or low-volume loss must not be disregarded except in the case of observed faulty machine operation.
11.4.4 Re-Qualification of Apparatus After the test appa- ratus has been initially qualified, it is required that one or more standard reference materials be periodically tested to ensure the accuracy of the data generated by the apparatus. This is particularly necessary when new test operators are involved or when the apparatus is not used on a regular basis. Re- qualification is also required for interlaboratory testing and for the qualification of materials as specified in customer and vendor contracts.
11.5 While two or more laboratories may develop test data that is within the acceptable coefficient of variation for their own individual test apparatus, their actual averages may be relatively far apart. The selection of sample size and the method for establishing the significance of the difference in averages shall be agreed upon between laboratories and shall be based on established statistical methods in Practices E 122, E 177, and Manual MNL 7.10
11.6 Reference Materials—Reference materials11 may be used for periodic monitoring of the test apparatus and proce- dures in individual laboratories.
11.6.1 While any of the four test procedures (Table 2) may be used on reference materials, it is recommended that Proce- dure A be used for the more abrasion-resistant materials such as AISI D-2 Tool Steel. When Procedure A volume loss values exceed 100 mm3 in materials such as annealed low-carbon steel, greater accuracy in material ranking can be obtained by using Procedures B or D.