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 procedures 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 Procedure 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.
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.