In the presence of an outlier, the t-test is not robust. For example, for two independent samples when the data distributions are asymmetric (that is, the distributions are skewed) or the distributions have large tails, then the Wilcoxon rank-sum test (also known as the Mann–Whitney "U" test) can have three to four times higher power than the "t"-test. The nonparametric counterpart to the paired samples "t"-test is the Wilcoxon signed-rank test for paired samples. For a discussion on choosing between the "t"-test and nonparametric alternatives, see Lumley, et al. (2002).