The Use of Z-scores in External Performance Evaluation of Inter-comparison Exercises

The inter-comparison exercise results carried as a part of external quality control helps in establishing the accuracy and uncertainty of the method and the measurement system. The performance evaluation is based on the statistical performance indicators such as precision, trueness, relative bias and z-score. Z-score is used for scaling the difference between a participant’s result and the assigned value for the concentration of the radio-analyte. It allows interpreting the data with different means and standard deviations and placing them on a common scale. This standard scale was used to compare result of the proficiency test made over the years for different radionuclides in matrices such as water, soil, sediment, seaweed, rice grain, having variable mean and standard deviation.