If the distribution of the residuals was judged as nonnormal by the preliminary Shapiro-Wilk test, the two
samples were compared by means of Mann-Whitney’s U test (Table 2, right). As for Strategy I, the Type I error
rate of the conditional U test was closest to the nominal α for samples from the exponential distribution. For
samples from the uniform distribution, the U test did not fully exhaust the significance level but showed remarkably
anti-conservative behavior for samples drawn from the normal distribution, which was most pronounced
in small samples and numerically low αpre.