Objective determination of defined characteristics of materials usually involves measurements based on a small proportion of the material of interest. Where materials are homogeneous, obtaining a representative sub-sample of the whole is a relatively simple problem. Where there is heterogeneity obtaining a sub-sample that is representative of the whole is a much more difficult task.

There are a several factors that bear on the technical and commercial application of objective measurement systems, the four most important of these are:

Sampling

Wool is a heterogeneous material, both in the bulk or when still on the sheep’s back. The sampling procedures for sale lots or consignments of wool have been carefully developed to ensure that the sample represents the bulk with a predictable degree of error. The requirements for sampling the bulk also extend to further sub-sampling of the sample itself, in order to measure a specific characteristic. Sampling is the first and most important step in any wool testing system.

Precision

Precision describes the reproducibility of results - that is the agreement between numerical values of two or more measurements determined using the same measurement systems. Common components of all measurement systems are:

• Sampling (both from the bulk and further sampling of samples obtained from the bulk)
• Instruments or machines
• Laboratories
• People

Each of these components introduces errors and the errors are additive. The precision of a measurement of any particular characteristic is therefore determined by the sum of the errors in each of these components of the measurement system.

Bias

Wool metrologists are concerned with two types of errors:

• Random or indeterminate errors
• Systematic or determinate errors

The error in the mean of a number of replicate measurements is equal to the sum of these two errors. Random or indeterminate errors impact upon precision. Bias may have little or no effect on precision, but it has a significant effect upon accuracy i.e. how close the measurement is to the “true” result.

Bias is a result of systematic or determinate errors. Systematic errors always act in one direction, resulting in a consistently larger or a consistently smaller result than the ”true” result. Bias can result from several causes, and generally, these can be classified into one of six groups:

• Sampling
• Differences in fundamental assumptions
• Personal errors
• Instrumental errors;
• Method errors; or
• Interferences.

Bias may be constant over the range of variation of the characteristic being measured, or it may vary over this range. One of the objectives of standardising wool testing systems is the elimination or at least the minimisation of bias. Where bias cannot be eliminated, provided it is not level dependent, a measurement technology may still be useful.

Cost

Industry adoption of any objective measurements is wholly influenced by the cost versus the benefit. Therefore, in establishing an objective measurement system, factors such as sampling, precision and bias must be balanced against the costs and the potential benefits. As a general rule, metrologists try to balance the demands of sampling, precision and bias to provide a measurement at an acceptable cost. This inevitably means that some compromises must be made.