26. Outliers: Muscular Endurance. The muscular endurance of 15 randomly selected males was measured after taking a single 600-milligram dose of vitamin C and also after taking a sugar placebo. Muscular endurance was measured by repetitive grip strength.⁶ The following data set represents the differences in muscular endurance (vitamin C – placebo).

The accompanying figure is a boxplot of the data. Note that the Wilcoxon signed rank test is a robust method (see Section 3.6 for similar robust methods) and is insensitive to the presence of the outlier indicated in the boxplot. In other words, the presence of a reasonable number of outliers does not impair the performance of the Wilcoxon signed rank test. Perform the Wilcoxon signed rank test at level of significance to determine whether the population median of the differences (vitamin C – placebo) differs from zero.