Total Youden

A reagent blank corrects the measured signal for signals due to reagents other than the sample that are used in an analysis. The most common reagent blank is prepared by omitting the sample. When a simple reagent blank does not compensate for all constant sources of determinate error, other types of blanks, such as the total Youden blank, can be used. 

The concept is illustrated in Figure 3.3 for data point seven of the example in Section 3.1. The total response (32.57) can be thought of as consisting of two parts - the contribution from the mean (32.53) and that part contributed by a deviation from the mean (0.04). Similarly, data point five of the same data set can be viewed as being made up of the mean response (32.53) and a deviation from the mean (-0.05), which add up to give the observed response (32.48). The residuals for the whole data set are shown in Figure 3.4. The set of residuals has n-1 degrees of freedom [Youden (1951)]. 

While methods validation and accuracy testing considerations presented here have been frequently discussed in the literature, they have been included here to emphasize their importance in the design of a total quality control protocol. The Youden two sample quality control scheme has been adapted for continuous analytical performance surveillance. Methods for graphical display of systematic and random error patterns have been presented with simulated performance data. Daily examination of the T, D, and Q quality control plots may be used to assess analytical performance. Once identified, patterns in the quality control plots can be used to assist in the diagnosis of a problem. Patterns of behavior in the systematic error contribution are more frequent and easy to diagnose. However, pattern complications in both error domains are observed and simultaneous events in both T and D plots can help to isolate the problems. Point-by-point comparisons of T and D plots should be made daily (immediately after the data are generated). Early detection of abnormal behavior reduces the possibility that large numbers of samples will require reanalysis. 

A reference method is one which after exhaustive investigation has been shown to have negligible inaccuracy in comparison with its imprecision [International Federation of Clinical Chemistry (IFCC), 1979]. With its comparison of inaccuracy and imprecision this definition clearly refers to the principles of quality control in clinical chemistry. Indeed, statistical models such as Youden plots are used to find out whether the error in a pair of results happens by chance (imprecision of the method) or is systematic (inaccuracy) (Youden, 1967). If the results are close to the true values, inaccuracy is negligible in comparison with imprecision. As demonstrated earlier, each analytical procedure has a certain degree of imprecision consequently, the total absence of systematic error can never be proved. Only as the influence of a systematic error is evident in comparison with the influence of chance or random error can the systematic error be demonstrated. 

From the beginning it was clear that it was essential to examine each step criticaUy in the methods for MeHg determination. To do so, several series of samples were prepared to (i) test the detection methods with solutions of known composition of Hg compounds, (ii) test the performance of the separation using cleaned extracts, (iii) verify the clean-up procedures by performing analysis of raw extracts and (iv) test the total analytical procedure by analyzing real samples. These evaluations were carried out in three intercomparisons, the first one of which started in 1987 (simple solutions), the second one in 1988 (cleaned and raw extracts) and the third round-robin exercise was carried out in 1989 (extracts and real samples) [81]. Youden plots [83] in some cases supported the technical conclusions. 

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