[R] wilcox.test - difference between p-values of R and online calculators

Thu Jan 22 01:18:35 CET 2015

David and Peter, I want to give a *very* belated thanks for your responses.
They were enlightening. I ultimately used the wilcox_test function from the
coin library.

Cheers,
Brad

____________________
W. Bradley Knox, PhD
http://bradknox.net
bradknox at mit.edu

On Wed, Sep 3, 2014 at 4:20 PM, peter dalgaard <pdalgd at gmail.com> wrote:

> Notice that correct=TRUE for wilcox.test refers to the continuity
> correction, not the correction for ties.
>
> You can fairly easily simulate from the exact distribution of W:
>
> x <- c(359,359,359,359,359,359,335,359,359,359,359,
>       359,359,359,359,359,359,359,359,359,359,303,359,359,359)
> y <- c(332,85,359,359,359,220,231,300,359,237,359,183,286,
>       355,250,105,359,359,298,359,359,359,28.6,359,359,128)
> R <- rank(c(x,y))
> sim <- replicate(1e6,sum(sample(R,25))) - 325
>
> # With no ties, the ranks would be a permutation of 1:51, and we could do
> sim2 <- replicate(1e6,sum(sample(1:51,25))) - 325
>
> In either case, the p-value is the probability that W >= 485 or W <= 165,
> and
>
> > mean(sim >= 485 | sim <= 165)
> [1] 0.000151
> > mean(sim2 >= 485 | sim2 <= 165)
> [1] 0.002182
>
> Also, try
>
> plot(density(sim))
> lines(density(sim2))
>
> and notice that the distribution of sim is narrower than that of sim2
> (hence the smaller p-value with tie correction), but also that the normal
> approximationtion is not nearly as good as for the untied case. The
> "clumpiness" is due to the fact that 35 of the ranks have the maximum value
> of 34 (corresponding to the original 359's).
>
> -pd
>
> On 03 Sep 2014, at 19:13 , David L Carlson <dcarlson at tamu.edu> wrote:
>
> > Since they all have the same W/U value, it seems likely that the
> difference is how the different versions adjust the standard error for
> ties. Here are a couple of posts addressing the issues of ties:
> >
> > http://tolstoy.newcastle.edu.au/R/e8/help/09/12/9200.html
> >
> http://stats.stackexchange.com/questions/6127/which-permutation-test-implementation-in-r-to-use-instead-of-t-tests-paired-and
> >
> > David C
> >
> > From: wbradleyknox at gmail.com [mailto:wbradleyknox at gmail.com] On Behalf
> Of W Bradley Knox
> > Sent: Wednesday, September 3, 2014 9:20 AM
> > To: David L Carlson
> > Cc: Tal Galili; r-help at r-project.org
> > Subject: Re: [R] wilcox.test - difference between p-values of R and
> online calculators
> >
> > Tal and David, thanks for your messages.
> >
> > I should have added that I tried all variations of true/false values for
> the exact and correct parameters. Running with correct=FALSE makes only a
> tiny change, resulting in W = 485, p-value = 0.0002481.
> >
> > At one point, I also thought that the discrepancy between R and these
> online calculators might come from how ties are handled, but the fact that
> R and two of the online calcultors reach the same U/W values seems to
> indicate that ties aren't the issue, since (I believe) the U or W values
> contain all of the information needed to calculate the p-value, assuming
> the number of samples is also known for each condition. (However, it's been
> a while since I looked into how MWU tests work, so maybe now's the time to
> refresh.) If that's correct, the discrepancy seems to be based in what R
> does with the W value that is identical to the U values of two of the
> online calculators. (I'm also assuming that U and W have the same meaning,
> which seems likely.)
> >
> > - Brad
> >
> > ____________________
> > W. Bradley Knox, PhD
> > http://bradknox.net<http://bradknox.net/>
> > bradknox at mit.edu<mailto:bradknox at mit.edu>
> >
> > On Wed, Sep 3, 2014 at 9:10 AM, David L Carlson <dcarlson at tamu.edu
> <mailto:dcarlson at tamu.edu>> wrote:
> > That does not change the results. The problem is likely to be the way
> ties are handled. The first sample has 25 values of which 23 are identical
> (359). The second sample has 26 values of which 12 are identical (359). The
> difference between the implementations may be a result of the way the ties
> are ranked. For example the R function rank() offers 5 different ways of
> handling the rank on tied observations. With so many ties, that could make
> a substantial difference.
> >
> > Package coin has wilxon_test() which uses Monte Carlo simulation to
> estimate the confidence limits.
> >
> > -------------------------------------
> > David L Carlson
> > Department 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>
> [mailto:r-help-bounces at r-project.org<mailto:r-help-bounces at r-project.org>]
> On Behalf Of Tal Galili
> > Sent: Wednesday, September 3, 2014 5:24 AM
> > To: W Bradley Knox
> > Cc: r-help at r-project.org<mailto:r-help at r-project.org>
> > Subject: Re: [R] wilcox.test - difference between p-values of R and
> online calculators
> >
> > It seems your numbers has ties. What happens if you run wilcox.test with
> > correct=FALSE, will the results be the same as the online calculators?
> >
> >
> >
> > ----------------Contact
> > Details:-------------------------------------------------------
> > Contact me: Tal.Galili at gmail.com<mailto:Tal.Galili at gmail.com> |
> > Read me: www.talgalili.com<http://www.talgalili.com> (Hebrew) |
> www.biostatistics.co.il<http://www.biostatistics.co.il> (Hebrew) |
> > www.r-statistics.com<http://www.r-statistics.com> (English)
> >
> ----------------------------------------------------------------------------------------------
> >
> >
> >
> > On Wed, Sep 3, 2014 at 3:54 AM, W Bradley Knox <bradknox at mit.edu<mailto:
> bradknox at mit.edu>> wrote:
> >
> >> Hi.
> >>
> >> I'm taking the long-overdue step of moving from using online
> calculators to
> >> compute results for Mann-Whitney U tests to a more streamlined system
> >> involving R.
> >>
> >> However, I'm finding that R computes a different result than the 3
> online
> >> calculators that I've used before (all of which approximately agree).
> These
> >> calculators are here:
> >>
> >> http://elegans.som.vcu.edu/~leon/stats/utest.cgi
> >> http://vassarstats.net/utest.html
> >> http://www.socscistatistics.com/tests/mannwhitney/
> >>
> >> An example calculation is
> >>
> >>
> >>
> *wilcox.test(c(359,359,359,359,359,359,335,359,359,359,359,359,359,359,359,359,359,359,359,359,359,303,359,359,359),c(332,85,359,359,359,220,231,300,359,237,359,183,286,355,250,105,359,359,298,359,359,359,28.6,359,359,128))*
> >>
> >> which prints
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >> *Wilcoxon rank sum test with continuity correction  data: c(359, 359,
> 359,
> >> 359, 359, 359, 335, 359, 359, 359, 359, 359, and c(332, 85, 359, 359,
> 359,
> >> 220, 231, 300, 359, 237, 359, 183, 359, 359, 359, 359, 359, 359, 359,
> 359,
> >> 359, 303, 359, 359, and 286, 355, 250, 105, 359, 359, 298, 359, 359,
> 359,
> >> 28.6, 359, 359) and 359, 128)  W = 485, p-value = 0.0002594 alternative
> >> hypothesis: true location shift is not equal to 0 Warning message: In
> >> wilcox.test.default(c(359, 359, 359, 359, 359, 359, 335, 359, : cannot
> >> compute exact p-value with ties*
> >>
> >>
> >> However, all of the online calculators find p-values close to 0.0025,
> 10x
> >> the value output by R. All results are for a two-tailed case.
> Importantly,
> >> the W value computed by R *does agree* with the U values output by the
> >> first two online calculators listed above, yet it has a different
> p-value.
> >>
> >> Can anyone shed some light on how and why R's calculation differs from
> that
> >> of these online calculators? Thanks for your time.
> >>
> >> ____________________
> >> W. Bradley Knox, PhD
> >> http://bradknox.net
> >> bradknox at mit.edu<mailto:bradknox at mit.edu>
> >>
> >>        [[alternative HTML version deleted]]
> >>
> >> ______________________________________________
> >> R-help at r-project.org<mailto:R-help at 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.
> >>
> >
> >        [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > R-help at r-project.org<mailto:R-help at 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.
> >
> >
> >       [[alternative HTML version deleted]]
> >
> > ______________________________________________
> > R-help at 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.
>
> --
> Peter Dalgaard, Professor,
> Center for Statistics, Copenhagen Business School
> Solbjerg Plads 3, 2000 Frederiksberg, Denmark
> Phone: (+45)38153501
> Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com
>
>
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