Improving Precision

As the cost of conducting tests is determined by the amount of sampling and testing, the final IWTO Method becomes a balance between the desirable level of precision and the cost of achieving it.

Some improvement in precision can be expected from better instrumentation and process control, but this will be a minor influence.

As the amount of testing on a lot has a direct relationship with precision, the simplest approach available to improve precision for commercial trading is to do additional sampling and testing. This may involve testing extra sub-samples or specimens, but most benefit will come from taking additional samples and testing extra sub-samples and specimens. The balance between increased cost and precision must be noted. Table 2 summarises the position of several strategies.

The greatest improvement is 42% and this is achieved when a lot is sampled and tested 3 times and each of the results combined. Again one must consider the cost versus the benefit. Considering the best precision strategies (42%), and a single lot with say 55% Wool Base; 3% VM Base; 21.0 um Mean Fibre Diameter; 90 mm Staple Length; and 30 N/kt Staple Strength, the improvement in precision would be:

At the time methods are developed and introduced, the adequacy of the precision is considered taking due regard to the cost and speed of service required.

From an exporter’s point of view, once lots are purchased and combined, they achieve a very precise set of results for processing consignments. Multiple-lot consignments have a significant improvement in precision when compared with a single test on the same consignment. Table 3 illustrates the effect of combination on the precision of results. It can be clearly seen that as the number of lots in a consignment increases, so does the precision of the result obtained.

Generally, both precision and accuracy are single lot problems and as such have potentially the greatest effect on the woolgrower who traditionally sells single lots. Major concern centres on fine wools where 0.1 um can make a substantial difference to the price paid or received. AWTA Ltd recognises that some growers may require results of higher precision and offers a multiple-sampling and testing service. However, generally growers do not have one test conducted in isolation but have their entire clip tested. The multiple lot testing ensures that, on average, the clip will have an improved precision compared to the individual lots.

The question of whether to split a lot prior to sampling and testing is of interest. There is no additional improvement in precision from splitting a lot prior to sampling and testing and testing each lot independently when compared with double sampling and testing the entire lot as one test. Both processes achieve an increase in precision of 29%. However, logistically it is easier to sample split lots, and it does offer opportunities to build sub-lots into Objectively Matched Lots or to sell the sub-lot in its own right if it slots into a currently favourable section of the market.

For the exporter who prepares blends and relies on a single test after blending, the risk to achieve the specification is greater. In these situations, small blended sub-lots should be sampled and tested individually, and then combined. In this way the improvement in precision identified in Table 3 can be obtained.