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

From: JP <jeanpaul.ebejer_at_inhibox.com>
Date: Thu, 28 Apr 2011 14:18:24 +0100

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
JP

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