[R] Orders of levels affecting wilcox.test() output

[David Carlson]

The short answer is whichever version you want since they are
the same. Look at the t-test results, they are also different.
In one t is positive and in the other it is negative. That is
just a result of whether the smaller mean is subtracted from
the larger mean or vice versa. The same is happening with
Wilcoxon (see
http://en.wikipedia.org/wiki/Mann-Whitney-Wilcoxon_test ) for
the specific equations. 

-------------------------------------
David L Carlson
Associate Professor of Anthropology
Texas A&M University
College Station, TX 77840-4352


-----Original Message-----
From: r-help-bounces at r-project.org
[mailto:r-help-bounces at r-project.org] On Behalf Of Alastair
Potts
Sent: Thursday, July 18, 2013 4:18 AM
To: r-help at r-project.org
Subject: [R] Orders of levels affecting wilcox.test() output

Good day all,

My first posting to this list. It looked like the best place
to post this
question.

When running the wilcox.test(), I noticed that the output
values change if
you change the ordering of the levels (example below which
includes a
t.test for comparison). I think this has something to do with
the change in
ranking order, but this doesn't make much sense as I would
expect the
ranking in this case to remain constant as there are no ties.
So my
questions are: a) is this correct (i.e. not a bug)?, and b) if
it is, then
which values should I report?


x <-
cbind(data.frame((c(rep("A",50),rep("B",50)))),runif(100))
colnames(x) <- c("a","b")
wilcox.test(b~a,data=x)
t.test(b~a,data=x)
x$a <- relevel(x$a,ref="B")
wilcox.test(b~a,data=x)
t.test(b~a,data=x)

Thank you in advance for your time,
Regards,
Alastair

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