In a recent paper, Cooper presented a nonparametric test for increasing (or decreasing) trend. This test is based upon comparisons of the order of all possible pairs of observations. Cooper assumes that the order of each pair is independent of the order of each other pair. Whenever two pairs have an observation in common, such an assumption is not valid. This changes the distribution of the statistic, which is equivalent to Kendall's tau. It also reduces its usefulness for small samples.
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References
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