Figure 3 shows similar power functions for exponential distributions,
where the Wilcoxon rank-sum test, the Wilcoxon signed-ranks test, and the
modified t test on ranks were substituted for the parametric t tests.
Comparison of the three power functions reveals that the outcome is almost
the same as in the case of the corresponding parametric tests applied to
normally distributed data. The Wilcoxon signed-ranks test was superior to
the Wilcoxon rank-sum test for paired data, while the modified t test on
ranks was slightly superior to both.