The p-value and inference is the same, the only difference that I see is
the value of the W statistic.  The definition of the W statistic (as
discussed in the Note section on the help page) is based on the sum of the
ranks in the **First** group, so if you change which group is first then
you should expect the W statistic to change.  As long as the inference does
not change, why worry about it?


On Thu, Jul 18, 2013 at 3:18 AM, Alastair Potts <potts.a@gmail.com> wrote:

> 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
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>



-- 
Gregory (Greg) L. Snow Ph.D.
538280@gmail.com

	[[alternative HTML version deleted]]

