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As a rule, the site pollution severity (SPS) is determined by the ESDD and NSDD of cap and pin standard disc insulator (reference insulator) under de-energized conditions. But now standard disc insulators are seldom used on transmission lines. In order to determine the SPS of certain districts, it is essential to figure out the coefficient of conversion from ESDD and NSDD of reference insulators to that of other profiles of insulators. Moreover, pollution condition prediction may require the quantification of insulator pollution characteristics. For the reasons above, a one-year-period natural pollution experiment of de-energized insulators was carried out in Henan, China. For the first time, the Dixon test is introduced to detect the outliers among the natural contamination data, which contributes to unveiling the data's law. The coefficient of conversion is defined as the relative contamination coefficient in the paper, which is based on the ratio of the contamination data of a certain type of insulator to that of the reference insulator. The Shapiro-Wilk test and normal quantile (Q-Q) plot testify the normality of the relative contamination values. Based on the normality of relative contamination values obtained in the experiment, the insulator pollution characteristic is quantified. Via the relative contamination coefficients, it is easy to judge the contamination accumulation difficulty between insulators and predict the pollution condition of most profiles of insulators. Moreover, a determination method of SPS is raised in the paper, which helps to reckon the pollution condition of one type of insulator on the basis of the known pollution condition of insulators of other types. Besides, the pollution discrepancy between the top and bottom surfaces of insulators is analyzed by the relative contamination values. [ABSTRACT FROM AUTHOR]