Consideration for evaluation method of proficiency test program on EMI measurement

Z-score based evaluation of proficiency testing for general tests is standardized in ISO 13528. Therefore, proficiency testing for EMI measurements is also evaluated with Z-score. Since Z-score is supposed to be calculated on the assumption that the distribution is normal distribution in the linear value, accordingly, the decibel values reported as EMI measurement readings shall be converted once to linear values before the calculation of Z-score. However, in this procedure, when the dispersion in data is large, the Z score calculated with the linear value is underestimated in case of the data smaller than a median value. This paper shows that Z score calculation by a decibel value has validity to the EMI testing. So, we concluded the Z score calculated with the decibel value gives the suitable evaluation results close to the distribution of raw data.