Abstract
The expression of the number of coli in samples of water from the results of analyses by the dilution method has been the subject of much discussion. Phelps (1) suggested the procedure of expressing the number as the reciprocal of the smallest portion in a geometrical series of dilutions which gives a positive test for coli always assuming that the dilutions of the sample are such that at least one portion of the series gives a negative result. This B. coli index, although readily calculated, is now generally recognized as an approximation. Its application becomes confusing when inconsistencies or so-called skips occur in the sequence or position of negative observations. Further difficulties are encountered in the estimation of the coli index whenever more than one observation is made at a given dilution. When an apparently anomalous, but nevertheless rationally probable, case occurs wherein a negative result is obtained in a dilution between two positives, then by this procedure the positive observation of the higher dilution is interchanged with the negative and the index of this newly arranged series stated in the usual way. Thus, as explained in ' 'Standard Methods for the Examination of Water and Sewage, 1925 edition, the interpretation of certain typical cases is as follows:
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