Assay number basic data

If participants in an external quality assessment scheme simply receive the results and check the performance index or some other measure to see if it is satisfactory, a great deal of valuable information is lost. Data from these schemes offer an opportunity to study a laboratory s assay and to assist in the education of laboratory personnel. Simple graphical approaches can be used that do not require complicated calculations. If a laboratory has a computer, more elaborate calculations can be carried out. Basically, the participating laboratory has analysed a sample for a particular analyte and has then to compare the value found with the True value. Possible estimates of the True value include the overall mean for all participants, the method mean, the mean value obtained by designated reference laboratories, or a stated spiked value. Some schemes provide all these values, but the majority provide an overall mean, which will be used in the following discussion. The overall mean has limitations if the technique for elimination of outliers is unsatisfactory or if a particular method, which causes a marked bias at certain concentrations in the overall mean, is used by a significant number of laboratories. 

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