5. Step 1: Specification

5.1. In the context of uncertainty estimation, "specification of the measurand" requires both a clear and unambiguous statement of what is being measured, and a quantitative expression relating the value of the measurand to the parameters on which it depends. These parameters may be other measurands, quantities which are not directly measured, or constants. It should also be clear whether a sampling step is included within the procedure or not. If it is, estimation of uncertainties associated with the sampling procedure need to be considered. All of this information should be in the Standard Operating Procedure (SOP).

5.2. In analytical measurement, it is particularly important to distinguish between measurements intended to produce results which are independent of the method used, and those which are not so intended. The latter are often referred to as empirical methods. The following examples may clarify the point further.

EXAMPLES:

  1. Methods for the determination of the amount of nickel present in an alloy are normally expected to yield the same result, in the same units, usually expressed as a mass or mole fraction. In principle, any systematic effect due to method bias or matrix would need to be corrected for, though it is more usual to ensure that any such effect is small. Results would not normally need to quote the particular method used, except for information. The method is not empirical.

  2. Determinations of "extractable fat" may differ substantially, depending on the extraction conditions specified. Since "extractable fat" is entirely dependent on choice of conditions, the method used is empirical. It is not meaningful to consider correction for bias intrinsic to the method, since the measurand is defined by the method used. Results are generally reported with reference to the method, uncorrected for any bias intrinsic to the method. The method is considered empirical.

  3. In circumstances where variations in the substrate, or matrix, have large and unpredictable effects, a systematic procedure is often developed with the sole aim of achieving comparability between laboratories measuring the same material. The method may then be adopted as a local, national or international standard on which trading or other decisions are taken, with no intent to obtain an absolute measure of the true amount of analyte present. Corrections for method bias or matrix effect are ignored by convention (whether or not they have been minimised in method development). Results are normally reported uncorrected for matrix or method bias. The method is considered to be empirical.

5.3. The distinction between empirical and non-empirical (sometimes called rational) methods is important because it affects the estimation of uncertainty. In examples 2 and 3 above, because of the conventions employed, uncertainties associated with some quite large effects are not relevant in normal use. Due consideration should accordingly be given to whether the results are expected to be dependent upon, or independent of, the method in use and only those effects relevant to the result as reported should be included in the uncertainty estimate.