On Wed, 2 Jan 2008, Prof Brian Ripley wrote: > Try exact = TRUE: the default switches to a normal approximation that will > not be adequate in your extreme example.
I've modified the code in R-devel to give an answer in such extreme cases, but note that the p-value is pretty inaccurate, and so is the coverage of the reported CI. > On Tue, 1 Jan 2008, Wolfgang Huber wrote: > >> When one of the two groups has only one member and the other one more >> than 49, wilcox.test will exit with the below error message, >> >> n=51; wilcox.test(1:n ~ 1:n==1, conf.int=TRUE) >> >> Error in uniroot(wdiff, c(mumin, mumax), tol = 1e-04, zq = >> qnorm(alpha/2, : >> f() values at end points not of opposite sign >> >> >> whereas with n=50 a result is returned: >> >> n=50; wilcox.test(1:n ~ 1:n==1, conf.int=TRUE) >> >> Wilcoxon rank sum test >> >> data: 1:n by 1:n == 1 >> W = 49, p-value = 0.04 >> alternative hypothesis: true location shift is not equal to 0 >> 95 percent confidence interval: >> 1 49 >> sample estimates: >> difference in location >> 25 >> >> >> I wonder whether it would be worthwhile to make the wilcox.test function >> handle such (admittedly pathologic) cases more gracefully. >> >> >> Happy New Year to all - >> Wolfgang >> >> ------------------------------------------------------------------ >> Wolfgang Huber EBI/EMBL Cambridge UK http://www.ebi.ac.uk/huber -- Brian D. Ripley, [EMAIL PROTECTED] 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 ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
