ASTM D2905-97

1.1 This practice provides a mechanism for calculating the number of specimens per laboratory sampling unit. It also provides recommended texts to be used in exceptional cases. Normally, it is preferable to specify in the section on sampling a small fixed number of specimens for each laboratory sampling unit. Occasionally, however, the task group writing a test method may think that the variability among the observations on specimens within a laboratory sampling unit will probably differ significantly from laboratory to laboratory. It is in unusual cases of this sort that the recommended texts in this practice should be used.

1.1.1 Paragraph A14.8 of the Recommendations on Form and Style specifies that statements on the number of specimens be included in all test methods.

1.2 This practice covers six optional recommended texts which serve as guides for the preparation of statements on the number of specimens required to determine the average quality of each unit of the laboratory sample under various conditions. The choice of the text to be used in a specific method will depend on the purpose of the test and the available information. This practice covers the following six different conditions.

1.3 Recommended Texts 1 through 4 are preceded by examples. Each example states part of the data from an interlaboratory test and illustrates the decisions needed in the course of calculating the numerical values required for inclusion in a specific text. Each of these texts describes two conditions: (1) the procedure to be followed when the user has a reliable estimate of the variability of the method in his own laboratory, and (2) the fixed number of specimens required when the user does not have a reliable estimate of the variability of the method in his own laboratory.

1.4 The instructions in this practice are specifically applicable to methods based on the measurement of variates. The instructions are not generally applicable to data based on attributes, and as a result, are not normally used for methods based on "go, no-go" tests.

1.5 This practice does not specify or discuss sampling plans but assumes that sampling has been adequately covered in other sections of the test method under preparation. However, to obtain the total number of specimens on which to base a decision to accept or reject a lot when acceptance testing a commercial shipment, multiply the number of specimens for each unit or the laboratory sample by the total number of such units in the entire lot sample. Instructions on the number of units in the laboratory sample to be taken from each of the primary sampling units in the lot sample should be in the applicable material specification or other agreement between the purchaser and the supplier.