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The MannWhitney UTest This page is available as a pdf download
Based upon material which may be found in Further Investigations in Biology (Vol. 1, 2, 3 and 4), Billiet, Casalis, Gaurenne & James, IBID Press.

STATISTICS
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Elementary Statistics Science usually works by taking samples rather than measuring every last item in a population. The mean of the sample you have selected is not necessarily the mean of the whole population. Nor is it necessarily true that if the sample means taken from two different populations are different then the population means of each must be different. There is bound to be a natural variation. The tests below determine how big the variation you obtain is in a sample.
Measuring the spread of the data Averages do not tell us everything about a sample. Samples can be very uniform with the data all bunched around the mean or they can be spread out a long way from the mean. The statistic that measures this spread is called the standard deviation. Arrange your data as follows and carry out the calculations:
(Note: ∑ = sum of ....)
You can use a programmable calculator or a spread sheet to do this calculation for you after you have entered the data. The standard deviation is a measure of the variation of the results. For data that is evenly distributed each side of the mean (a normal distribution) 68% of the data lies within one standard deviation of the mean.
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