Does calculation of the 95th percentile of microbiological results offer any advantage over percentage exceedence in determining compliance with bathing water quality standards?

Aims: Draft WHO guidance is likely to suggest a calculated 95th percentile method, rather than percentage exceedence, in assessing compliance of bathing waters with microbiological standards. This study set out to determine whether this was an appropriate development. Methods and Results: A series of Monte Carlo studies compared five non-parametric methods for calculating the 95th percentile with the parametric method and compares results with percentage exceedence. It is shown that the Hazen method gives the closest proximity to the parametric method for calculating 95th percentile values. However, the difference between 95th percentile results and percentage exceedence, as currently used, is trivial compared to uncertainty due to sample variation. Conclusions: It is concluded that a calculated 95th percentile for beach classification offers little advantage compared to percentage exceedence, other than offering a false sense of certainty. Furthermore, the additional calculation needed in determining 95th percentile values will demand electronic calculation, increase the chance of calculation errors and make the results less understandable to beach managers and the general public. Significance and Impact of the Study: This study should encourage the European Commission to keep percent compliance rather than calculation of 95th percentiles in any new bathing water directive.