The Precision of Microscopic Counts of Plankton in Water

Abstract

of microscopic organisms in water form a useful and informative part of water analysis, which finds application in many aspects of water, sewage, and waste control. A substantial section of Standard Methods (1) is devoted to the technique of microscopic counts. Comparatively little attention, however, seems to have been given to the precision of the microscopic counts conducted according to the directions set forth. In The Microscopy of Drinking Water (2) it is stated the results of counts made by the SedgwickRafter method are usually precise within 10 per cent; i.e., two examinations of the same sample seldom differ by more than this amount. No person experienced in making microscopic counts has ever felt too certain of the validity of this statement. The procedural sources of error, such as errors of sampling and sand error, are discussed in some detail in Microscopy. Of these, the error of random sampling from a large body of water is undoubtedly the greatest, and must itself often exceed the given figure of 10 per cent. Some slight mention is made of errors due to unequal distribution of organisms in the counting cell, but it is said that these counting errors cannot be readily stated in figures. With modern statistical methods, however, the counting errors can be determined directly from tabulated results. This article