A Research Team Investigated the Protective Effect of Two Variants

Multiple Choice

A research team investigated the protective effect of two variants of a vaccine against the simian immunodeficiency virus (SIV) in rhesus monkeys. They measured the viral load (in log scale) of monkeys randomly assigned to either a vaccine variant or a sham (fake) vaccine. Here is a partial software output for a Kruskal-Wallis test on the data:
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$$\begin{array}{l}\text { Kruskal-Wallis Test on viral load }\\\begin{array} { l r r r r } \text { vaccine } & \mathrm { N } & \text { Median } & \text { Ave Rank } & Z \\1 & 16 & 5.530 & 18.6 & - 0.18 \\2 & 13 & 5.030 & 11.2 & - 3.25 \\\text { sham } & 8 & 6.860 & 32.5 & 3.98 \\\text { Overall } & 37 & & 19.0 & \\H = 19.29 & \mathrm { DF } = \quad \mathrm { P } = \\\mathrm { H } = 19.30 \quad \mathrm { DF } = \quad \mathrm { P } = \quad \text { (adjusted for ties) }\end{array}\end{array}$$ Under the null hypothesis that the three populations have the same continuous distribution, which type of distribution does the Kruskal-Wallis statistic, H, have?

A) Approximately a chi-square distribution with 2 degrees of freedom
B) Approximately an F(2, 34) distribution
C) Approximately the standard Normal distribution
D) A distribution that cannot be evaluated because the populations may not be Normal