Electrical Measurements of Nerve Activity Give a Clue to the Nature

Multiple Choice

Electrical measurements of nerve activity give a clue to the nature of poisoning with the insecticide DDT. When a nerve is stimulated, its electrical response shows a sharp spike, which is followed by a much smaller second spike. In a randomized experiment, researchers measured the relative height of the second spike as a percent of the first spike, comparing six rats poisoned with DDT and a control group of six unpoisoned rats. Here are the findings: $$\begin{array} {| l | l l l l l l |} \hline \text { Poisoned } & 8.456 & 12.207 & 16.869 & 20.589 & 22.429 & 25.050 \\\hline \text { Unpoisoned } & 6.642 & 8.182 & 9.351 & 9.686 & 11.074 & 12.064 \\\hline\end{array}$$
Let W, the Wilcoxon rank sum test statistic, be the sum of the ranks assigned to the relative spike height in rats poisoned with DDT.
What is the standardized value z of the Wilcoxon rank sum test, W?

A) 0.16
B) 2.24
C) 6.24
D) 9.10