# The Bathing Water Regulations 2013, Paragraph 2: Methodology


(1) In this Schedule, “percentile value” is based on a percentile evaluation of the log10 normal probability density function of microbiological data used for the assessment under regulation 10.
(2) The appropriate agency must derive a percentile value as follows—
(a) take the log10 value of all bacterial concentrations in the data sequence to be evaluated or, if a zero value is obtained, take the log10 value of the minimum detection limit of the analytical method used;
(b) calculate the arithmetic mean (“μ”) of the log10 values taken under paragraph (a);
(c) calculate the standard deviation (“σ”) of the log10 values taken under paragraph (a);
(d) derive the upper 90-percentile point of the data probability density function from the following equation: upper 90-percentile = antilog (μ + 1.282 σ); and
(e) derive the upper 95-percentile point of the data probability density function from the following equation: upper 95-percentile = antilog (µ + 1.645 σ).

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Source: https://www.legislation.gov.uk/uksi/2013/1675/schedule/5/paragraph/2 (the official version)
Page: https://openlegislation.uk/bathing-water-regulations-2013/schedule-5/paragraph-2
Contains public sector information licensed under the Open Government Licence v3.0. Source: legislation.gov.uk, The National Archives.