The plate count method for estimating bacterial populations is satisfactory for many comparative purposes if relative rather than absolute numbers of cells are wanted, although in some cases, because of clumping, plate counts may not bear a constant relation to total counts even during the logarithmic growth phase (Jennison, 1937). This lack of agreement may be overcome, at least with some organisms, by proper shaking to break up clumps of cells (Ziegler and Halvorson, 1935).2 Considering the plating method per se, the total error of the mean plate count of a given dilution of cells is chiefly made up of two rather distinct sources of deviations: (a) the distribution or sampling error, sometimes inaccurately called the counting error, (i.e., variation in number of colonies, due to sampling, between replicate plates of the given dilution), and (b), the dilution error, (i.e., the errors of pipetting involved in reaching the given dilution). For purposes of discussion, it is assumed that optimum conditions are provided for the growth of the organisms. It is customary to measure the reliability of the plate count
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