Abstract: The calculation of percentiles proposed in the Directive 2006/7/EC (parametric approach) to evaluate bathing water quality uses two parameters: mean (µ) and standard deviation (?). These two parameters are good descriptors of data populations only when data are log normally distributed. Several previous studies have shown that a log transformation is sufficient to achieve normality, while other studies suggest that log normality in bathing water quality datasets is seldom attained. In our study, log normality was achieved in 59.6% of the cases. In order to try to obtain a transformation parameter for Box-Cox (?) that provides the best fit and perhaps normality in bathing water datasets, the maximum likelihood estimation (MLE) method was applied to 40.4% of the remaining (non log normal) datasets. Results show that there is no transformation parameter that ensures normality for all datasets. In fact, normality is only reached in 10.3% of these datasets but, in these cases, the parametric approach seems to be a good one to evaluate bathing water quality. In cases where normality was not fulfilled even by application of the MLE method, a non-parametric approach to calculate percentiles is considered the most appropriate one. When percentile values obtained through the parametric and non-parametric Hazen approaches are compared, it is shown that the percentage of bathing waters changing their classification is low (12.3%). In these cases, the Hazen approach provides the worst classification in a vast majority of cases (90.6%), being this change important in some cases, in which classification is downgraded from having ?Excellent? to ?Sufficient? quality. Therefore, the Hazen approach is more appropriate for calculating percentiles, since it provides better estimators of percentile values. Furthermore, this method involves a more conservative approach for the classification of bathing water quality, providing an additional security for bathers? health. The fact that normality is not fulfilled and that classification of bathing waters could change must be considered by policymakers in order to adopt an alternative method for evaluating bathing waters quality.

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