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The Air Quality Regulations 1997

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8.—(1) The nth percentile (that is to say the 99th or the 99.9th percentile) is calculated for a calendar year (yielding one result per calendar year) as follows:

  • Let:

(1) N equal the number of measurements for that calendar year;

(2) k equal—

rounded to the nearest whole number (or rounded up to the nearest whole number where the fractional part of the number is 0.5); and

(3) the N measurements be listed in ascending order:

  • X1; X2; X3; ... ... ... ... Xk; ... ... ... ... XN1; XN

  • Then: the nth percentile for that calendar year is the value of the measurement listed in the kth position; that is Xk.

(2) For the purpose of the calculation of the nth percentile, the measurements for a calendar year are:

(a)in the case of the 99th percentile of daily maximum running 24 hour means, the 365 daily maximum running 24 hour means for the calendar year in question (or 366 in the case of a leap year);

(b)in the case of the 99.9th percentile of 15 minute means, the 35040 15 minute means for the calendar year in question (or 35136 in the case of a leap year).

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