trouble with wilcox.test
If this is stack overflow (and I don't know that yet: when I tried this on Windows the traceback was clearly corrupt, referring to bratio), the issue is that it is impossible to catch such an error, and it is not even AFAIK portably possible to find the stack size limit (or even the current usage) to do some estimates. (The amount of RAM is not relevant.) On Unix-alikes the stack size limit can be controlled from the shell used to launch R so we don't have any a priori knowledge. The underlying code could be rewritten not to use recursion, but that seems not worth the effort involved. All I can see we can do it to put a warning in the help file.
On Thu, 18 Aug 2005, P Ehlers wrote:
Prof Brian Ripley wrote:
On Wed, 17 Aug 2005, Greg Hather wrote:
I'm having trouble with the wilcox.test command in R.
Are you sure it is not the concepts that are giving 'trouble'? What real problem are you trying to solve here?
To demonstrate the anomalous behavior of wilcox.test, consider
wilcox.test(c(1.5,5.5), c(1:10000), exact = F)$p.value
[1] 0.01438390
wilcox.test(c(1.5,5.5), c(1:10000), exact = T)$p.value
[1] 6.39808e-07 (this calculation takes noticeably longer).
wilcox.test(c(1.5,5.5), c(1:20000), exact = T)$p.value
(R closes/crashes) I believe that wilcox.test(c(1.5,5.5), c(1:10000), exact = F)$p.value yields a bad result because of the normal approximation which R uses when exact = F.
Expecting an approximation to be good in the tail for m=2 is pretty unrealistic. But then so is believing the null hypothesis of a common *continuous* distribution. Why worry about the distribution under a hypothesis that is patently false? People often refer to this class of tests as `distribution-free', but they are not. The Wilcoxon test is designed for power against shift alternatives, but here there appears to be a very large difference in spread. So
wilcox.test(5000+c(1.5,5.5), c(1:10000), exact = T)$p.value
[1] 0.9989005 even though the two samples differ in important ways.
Any suggestions for how to compute wilcox.test(c(1.5,5.5), c(1:20000), exact = T)$p.value?
I get (current R 2.1.1 on Linux)
wilcox.test(c(1.5,5.5), c(1:20000), exact = T)$p.value
[1] 1.59976e-07 and no crash. So the suggestion is to use a machine adequate to the task, and that probably means an OS with adequate stack size.
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One could also try wilcox.exact() in package exactRankTests (0.8-11) which also gives (with suitable patience) [1] 1.59976e-07 even on my puny 256M Windows laptop. Still, it might be worthwhile adding a "don't do something this silly" error message to wilcox.test() rather than having it crash R. Low priority, IMHO. Windows XP SP2 "R version 2.1.1, 2005-08-11" Peter Ehlers
Brian D. Ripley, ripley at stats.ox.ac.uk Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595