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 F1(µ + 1.645 σ).

Annotations

Textual amendments

F1: Sch. 5 para. 2(2)(e): formula substituted (21.11.2025 for E., 1.4.2026 for W.) by The Bathing Water (Amendment) (England and Wales) Regulations 2025 (S.I. 2025/1129), regs. 1(2)(3), 12