Definition of assigned values for proficiency tests in water analysis

The definition of an assigned value is usually achieved by calculating mean values from the data (with different methods) or by designating reference laboratories. Neither method is completely satisfactory. In this paper a ne w method is presented for the definition of the assigned value for spiked s amples with an unknown content of the analyte in the matrix. The method con sists of two parts. The first is the estimation of the assigned values from the spiked amounts and the content in the matrix, based on the results of reference laboratories. The other is the designation of these reference lab oratories by comparing their results with the assigned values. Because each of these parts requires the other, an iterative procedure is necessary. As an example, the results of a proficiency test for the analysis of copper i n wastewater are used to compare the calculated values with those from othe r methods, e.g., the Huber estimation.