[R] Simple General Statistics and R question (with 3 line example) - get z value from pairwise.wilcox.test

JP jeanpaul.ebejer at inhibox.com
Thu Apr 28 15:18:24 CEST 2011

Hi there,

I am trying to do multiple pairwise Wilcoxon signed rank tests in a
manner similar to:

a <- c(runif(1000, min=1,max=50), rnorm(1000, 50), rnorm(1000, 49.9,
0.5), rgeom(1000, 0.5))
b <- c(rep("group_a", 1000), rep("group_b", 1000), rep("group_c",
1000), rep("group_d", 1000))
pairwise.wilcox.test(a, b, alternative="two.sided",
p.adj="bonferroni", exact=F, paired=T)

This gives me the following output:

        group_a group_b group_c
group_b <2e-16  -       -
group_c <2e-16  0.25    -
group_d <2e-16  <2e-16  <2e-16

(which is kind of expected since group_b and group_c have similar distributions)

I have found that when doing a wilcoxon signed ranked test you should report:

- The median value (and not the mean or sd, presumably because of the
underlying potential non normal distribution)
- The Z score (or value)
- r
- p value

My questions are:

- Are the above enough/correct values to report (some places even
quote W and df) ?  What else would you suggest?
- How do I calculate the Z score and r for the above example?
- How do I get each statistic from the pairwise.wilcox.test call?

Many Thanks

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