Significance and Use
Assigning a value to any property of the material in a container or in a lot, consignment, or delivery involves a measurement process that includes both sampling and testing procedures. The correctness of the value assigned depends upon the variability due to testing. Even when the variability due to testing is minimized by carefully developed procedures, correct and consistent estimates of the true value of the property are possible only when the sampling procedure avoids systematic bias, minimizes variations due to sampling, and provides a laboratory sample of adequate size.
This practice may not give the most efficient sampling plan that might be devised in special situations but it does present a general procedure that gives satisfactory precision with an economical amount of sampling and one which does not require elaborate statistical computation based on previous knowledge of the amount of variation between lot samples, between laboratory samples, and between test specimens.
The smallest number of specimens required for a given variability in the average result will usually be obtained by (1) minimizing the number of shipping units in the lot sample, (2) taking one of the shipping units in the laboratory sample, and (3) taking the prescribed specimen(s) from the selected laboratory sample shipping unit. (See 7.3 and 7.4.)
To minimize the cost of sampling a lot of material, it is necessary to agree on the required variance for the reported average for a lot of material:
Estimate the variance due to lot samples, the variance due to laboratory samples, and the variance due to test specimens.
Calculate the total variance for the average test results for several combinations of the number of lot samples, the number of laboratory samples per lot sample, and the number of test specimens per laboratory sample.
Calculate the cost of performing each of the sampling schemes considered in 5.4.2.
Select the sampling scheme that (1) has the required precision, and (2) is most economical to perform.
1.1 This practice covers a procedure for the division of shipments of manufactured staple fiber, sliver (or top) or tow into lots and the sampling of such lots for testing.
Note 1—For sampling yarns, refer to Practice D2258.
Note 2—This practice differs from BISFA rules for staple fibers in the lot sampling, by the elimination of separate sampling of outer versus inner container areas, in the reduction of number of strata from 6 to 5, and by the elimination of compositing to obtain a single laboratory sample for the lot when testing properties which do not depend on as-received moisture content.
1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.