[R] wilcox.test - difference between p-values of R and online calculators
Tal Galili
tal.galili at gmail.com
Wed Sep 3 12:23:53 CEST 2014
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?
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On Wed, Sep 3, 2014 at 3:54 AM, W Bradley Knox <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
>
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>
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