The Minimum Number of Acceptable Subsample Results Required in an IWTO Certificate for Wools of UK Origin

Abstract

The IWTO Core Test Regulations currently stipulate that a minimum of three subsamples shall be tested when conducting tests on wools of UK origin. Using data from more than 200 randomly-selected commercial lots, this report has examined the impact of using two subsamples for the calculation of test results.

It has been demonstrated that the accuracy of testing is not jeopardised when two subsamples, instead of three, are used to calculate Wool Base and VM Base results. The average difference between results based on two and three subsamples was 0.017% and 0.000% for Wool Base and VM Base respectively. The variability of the differences for both tests was small and individual differences were independent of the measured result. The method of drawing test specimens for fibre diameter and colour measurement ensures test results for these parameters are not affected by the number of subsamples tested.

The impact on test precision of using a two-subsample testing regime was also shown to be minimal. The 95% Confidence Limits from two-subsample testing procedures were approximately 0.06% larger than those calculated from three subsamples.

With technical equivalence demonstrated between two subsample and three subsample testing processes, and improved service times and reduced testing fees achievable using a two-subsample testing regime for UK wools, an amendment to the current requirement for the minimum number of subsamples for wools of UK origin is recommended. 

Citation

"The Minimum Number of Acceptable Subsample Results Required in an IWTO Certificate for Wools of UK Origin", D. G. Knowles, Submission 02, Raw Wool Group, IWTO Evian Meeting, May 2004