Summary. The device for splitting a plankton sample into two approximately equal parts consists of a hollow cylindrical drum mounted to turn on a horizontal axis, and a vertical semi-circular septum cemented into place midway between the end walls of the drum. About a quart wilt fill the drum up to the axis. After rotating the drum until the septum is above, the sample is poured in. Then the drum is rotated until the septum splits the sample. Lifting the drum ahd rotating a little more, the two separated samples are drained into containers of convenient shape for emptying and filling. Smaller samples are obtained by emptying one container into the drum. Thus aliquot portions o F approximately 1/2 , 5/4, 1/s , etc. of the original are obtained, and the process is continued until the sample is small enough for counting. In general the sample consists of a number of different kinds of plankters each of which is counted separately. Multiplying each count in the ruth fraction by 2m gives an estimate of the number in the original sample. Each step in the splitting process is subject to random errors and possibly biased errors. Studies of these errors are based upon a series of test runs involving both volume measurements and plankton counts. The samples split were specially chosen to represent different types of plankton communities.The several types tested proved not to influence the results materially.Statistical tests involving distribution of Z 2 and departure from the mean, of left and right portions gave convincing evidence that the errors .were random.Thus, averaging separate counts would be expected to reduce the error. Researches on the magnitude of the error were based upon the law of propagation of errors of a product. Furthermore a study of the difference between the left and right samples as well as other investigations on the different sources of error have been made.
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