Hi, In Wilcoxon test , we look for medians rather than the means. Ratio of medians should be more coherent with P value.
On Thu, Sep 20, 2012 at 6:30 PM, Ben Bolker <bbol...@gmail.com> wrote: > Mohamed Radhouane Aniba <aradwen <at> gmail.com> writes: > > > > > > > Thank you Thomas, > > > > So you think a t-test is more adequate to use in this case ? > > > > Rad > > No, because a t-test makes even stronger parametric assumptions. > You were given more specific advice on stackoverflow > http://stackoverflow.com/questions/12499687/wilcoxon-test-and-mean-ratios > If you want to prove that there is *some* difference between the > distributions, you're done. If you want to test for some specific > difference, you need to think more about what kind of test you want > to do. Permutation tests with various test statistics are a way > to approach that. > > Ben Bolker > > > > > > On Sep 19, 2012, at 8:43 PM, Thomas Lumley <tlumley <at> uw.edu> wrote: > > > > > On Thu, Sep 20, 2012 at 5:46 AM, Mohamed Radhouane Aniba > > > <aradwen <at> gmail.com> wrote: > > >> Hello All, > > >> > > >> I am writing to ask your opinion on how to interpret this case. I > have two > vectors "a" and "b" that I am trying > > to compare. > > [snip] > > > > > > > There's nothing conceptually strange about the Wilcoxon test showing a > > > difference in the opposite direction to the difference in means. It's > > > probably easiest to think about this in terms of the Mann-Whitney > > > version of the same test, which is based on the proportion of pairs of > > > one observation from each group where the `a' observation is higher. > > > Your 'c' vector has a lot more zeros, so a randomly chosen observation > > > from 'c' is likely to be smaller than one from 'a', but the non-zero > > > observations seem to be larger, so the mean of 'c' is higher. > > > > > > The Wilcoxon test probably isn't very useful in a setting like this, > > > since its results really make sense only under 'stochastic ordering', > > > where the shift is in the same direction across the whole > > > distribution. > > > > > > -thomas > > > > > > -- > > > Thomas Lumley > > > Professor of Biostatistics > > > University of Auckland > > ______________________________________________ > 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. > -- Avinash Barnwal, M.Sc. Statistics and Informatics Department of Mathematics IIT Kharagpur [[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.
