The study was conducted to evaluate the z-scores of accredited and non-accredited laboratories for the electrochemical analysis namely pH, conductivity and salinity through an interlaboratory comparison test carried out on real wastewater samples. The participants were small scale 16 non-accredited and 9 accredited wastewater analysis laboratories in Turkey. The samples from a wastewater treatment plant were first collected from influent and effluent lines. The third sample was prepared by mixing the influent and effluent water samples to obtain three different levels of analysis as high, low and medium, respectively. pH, conductivity or salinity are the measurements based on electrochemical analytical methods in which the influence of the other factors such as technical staff ability on the measurement is limited. However, the method used in the z-score calculation could have a substantial effect on the z-score performance of a laboratory. Therefore, three different z-score calculation approaches using standard deviation (SD) for proficiency assessment and different assigned values were applied to the interlaboratory comparison test results. The assigned value was determined through average, and median of individual groups named as accredited or non-accredited laboratories. The results indicated that the method used in the z-score calculation does not substantially affect the scores of the laboratory. No population quality effect on the z-score exists. The laboratory with low performance will not pass the comparison test whatever the z-score calculation method.
Intelaboratory comparison test, Conductivity, , Salinity, pH, Salinity, Wastewater, z-score