The Assigned Value

All proficiency testing schemes should have a statistical protocol which states clearly how the data will be processed and how laboratory performance will be evaluated. This protocol should also describe how the assigned value for any parameter in a test sample is estimated. This is an important consideration, as the performance of individual laboratories is gauged by comparison with the assigned value. 

The assigned value is the value attributed to the quantity being measured in the proficiency testing exercise. It can be considered the target value for participants. There are a number of different approaches to determining the assigned value. These are by use of  

The approach is chosen by the scheme organizer, in consultation with the members of the scheme. Each approach has advantages and disadvantages, as discussed in the following sections. 

In this context, formulation involves the preparation of synthetic test materials. A known amount or concentration of the analyte is added to a suitable material (i.e. the sample matrix) containing none of the analyte of interest (or to a material containing a very small but well characterized amount of the analyte). Such 

However, there are a number of problems associated with this approach. It may be difficult to produce material of sufficient homogeneity owing to problems in mixing the analyte with the sample matrix. It may also be difficult to obtain a suitable blank or sufficiently well characterized test material. In addition, a synthetic material may well behave differently from a natural material. The analyte may be in a different chemical form or less strongly bound to the sample matrix. For these reasons, the formulation approach is not appropriate for many sample types. 

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Defining fixed points of the International Temperature Scale of 1990 (ITS-90). Except for the triple points, the assigned values of temperature are for equilibrium states at a pressure of one standard atmosphere (101 325 Pa). 

The data obtained from the resistance measurements are shown in Fig. 4.10. The assigned values of conductivity are limited in accuracy because the measured resistance was found to be somewhat time dependent. The [100] datum at the lowest strain was particularly so and a definite resistance value cannot be assigned to that point. 

It is necessary to specify two conditions for the complete posing of this or that problem. The assigned values of y and Ay suit us perfectly and lie in the background a widespread classification which will be used in the sequel. When equation (6) is put together with the values yi and A yi given at one point, they are referred to as the Cauchy problem. Combination of two conditions at different nonneighboring points with equation (6) leads to a boundary-value problem. 

When the isotermic flow of the ideal gas (17) is considered, scheme (43) can be written in simplified form, since the energy equation was disappeared because T = const. A proper iteration process is governed by the same rule as in the adiabatic case, the convergence of which can be established in a similar way without difficulties. In the isotermic case with the assigned values 7 = 1, a = lOo, 6 = 0 we deduce instead of (58) that... 

After that, Newton s method of iterations applies equally well to either of these groups independently. By analogy with the isotermic case the first group of equations is to be solved with a prescribed temperature, while the second one needs the assigned values of rj and v. The essence of the matter in the last case is that the origin of the heat conduction equation is stipulated by the available sources of a dynamical nature. 

Upon substituting the assigned value into (39) we derive the formula k k k k... 

The three historical approaches to certification mentioned above were recently expanded to identily seven modes that are used at NIST for value assignment for chemical composition (May et al. 2000). These seven modes and the resulting values are summarized in Table 3.13. The basic principles of value assignment remain unchanged however, these modes now provide a well-defined link between the process used for value assignment and the definition of the assigned value (i.e. certified, reference, or information value). The terms described above provide a clear indication of the level of confidence that NIST has in the accuracy of the assigned value. The definition of a certified value implies that NIST must be involved in the measurement process for the value to be classified as a NIST certified value (see modes 1-3 in Table 3.13). Thus, modes 4 and 7, which do not involve NIST measure-... 

Summation of the results of the determinations of water, organic solvents, mineral impurities and the organic components amounts to loo %. The results of methods employed to analyze the substance other than these given above are not used for the calibration of the assigned value but are reported to support the results obtained by the defining methods. For most reference substances intended as assay standards the assigned content is normally expressed as is so that it is essential (when establishing the CRS) to determine the content of water and residual solvents for a non-specific assay and also, for a selective assay, to determine the content of impurities. 

In the specific case of the European Pharmacopoeia reference substances, it is considered that the recommendation in the ISO guidelines, to give the uncertainty value with the assigned value on the label of the CRS, is not relevant because of the following reasons ... 

Reference Substances and Spectra for Pharmaceutical Analysis 1185 Tab. 54 Uncertainty of the assigned value of ciprofloxacin HCl CRS 2... 

If the risk of rejecting a good result is considered to be acceptable, there is no justification of giving uncertainty values with the assigned value. 

The principle of proficiency testing schemes consists in analyzing one or more samples sent to the laboratories by an external body. The analytical results returned to the organizer are evaluated in comparison to the assigned value(s) of the sample(s). 

To be familiar with the different approaches to obtaining the assigned value and target range for a proficiency testing scheme. 

Provided the sample matrix and analyte concentration are appropriate, matrix Certified Reference Materials (CRMs) can make ideal proficiency testing samples. The assigned value is the certified value given on the certificate accompanying the CRM. The certificate will also give an uncertainty estimate for the certified value, and the use of CRMs allows the traceability of analytical data to be established. However, matrix CRM availability is limited and the materials are often expensive. Hence, Certified Reference Materials are seldom used as PT samples. 

In this case, the assigned value is obtained from data produced by a number of expert laboratories who have analysed the proficiency testing sample by using a recognized reference method. The laboratories must be able to demonstrate their... 

A consensus value is the easiest and cheapest method for obtaining the assigned value. It is the appropriate approach when all participants are using a single standardized empirical method and a large number of laboratories are involved. 

The z-score takes no account of the uncertainties in the assigned value or the participants results. The z -score is similar to the z-score, but takes into account the uncertainty in the assigned value, ux, as shown in the following equation ... 

Therefore, z -scores will only differ significantly from z-scores if there is significant uncertainty in the assigned value. 

There are a number of other scoring systems but these are not as widely used as the z-score system. En numbers take into account the expanded uncertainty in the assigned value (Um ) and the expanded uncertainty in the participant s result... 

En numbers are used when the assigned value has been produced by a reference laboratory, which has provided an estimate of the expanded uncertainty. This scoring method also requires a valid estimate of the expanded uncertainty for each participant s result. A score of En < 1 is considered satisfactory. The acceptability criterion is different from that used for z-, z - or zeta-scores as En numbers are calculated using expanded uncertainties. However, the En number is equal to zeta/2 if a coverage factor of 2 is used to calculate the expanded uncertainties (see Chapter 6, Section 6.3.6). En numbers are not normally used by proficiency testing scheme providers but are often used in calibration studies. 

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