Estimation of bacterial density of water samples; methods of attaining international comparability.

The statistical problems involved in the estimation of the bacterial density of water samples by the so-called "dilution" method are reviewed, and some ways of arriving at international comparability of estimates are suggested. It is emphasized that the aim of the dilution test should be not only to determine the "most probable number" of organisms in the sample, but also to specify the accuracy of the estimation by providing two confidence limits within which the true bacterial density will lie. The dilution schemes recommended for routine testing of water quality, for more precise estimations of the number of organisms in a water supply before and after treatment, at different times, or at different levels, and for tests of heavily polluted supplies are described, and the factors affecting the frequency with which sampling should be carried out are briefly discussed.