R help - Dec 2009 - Problem with "Cannot compute correct p-values with ties"

Dear All,
   1. why did the problem happen?
   2. How to solve it?

   --

Best wishes,
Zhijiang Wang
--------------------------------------------
PHD Student
Room 212, Science buliding,
The International WIC Institute,
College of Computer Science and Technology,
Beijing University of Technology,
Beijing, China.

On Wed, 02 Dec 2009 16:52:23 +0800 Zhijiang Wang 
<ivorytower at emails.bjut.edu.cn> wrote:
>    Dear All,
>    1. why did the problem happen?
>    2. How to solve it?
A good start would be to follow the advice at the bottom of every 
posting on this mailing list:

PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

-- 
Karl Ove Hufthammer

Please try to find answers by yourself
You should not just post problems without thinking them.

                                                                             
             Huang, Guo-Hao

--------------------------------------------------
From: "Zhijiang Wang" <ivorytower at emails.bjut.edu.cn>
Sent: Wednesday, December 02, 2009 4:52 PM
To: <R-help at r-project.org>
Subject: [R] Problem with "Cannot compute correct p-values with ties"


   Dear All,
   1. why did the problem happen?
   2. How to solve it?

   --

Best wishes,
Zhijiang Wang
--------------------------------------------
PHD Student
Room 212, Science buliding,
The International WIC Institute,
College of Computer Science and Technology,
Beijing University of Technology,
Beijing, China.
______________________________________________
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.

I am sorry that I didn't express my question more clearly and made you
   understand it so difficult.
   But thank you all your kind help very much and I have used another method
   for solving it.
   Thank you all again!
   [1]guohao.huang at gmail.com ??????:

Please try to find answers by yourself
You should not just post problems without thinking them.


             Huang, Guo-Hao

--------------------------------------------------
From: "Zhijiang Wang" [2]<ivorytower at emails.bjut.edu.cn>
Sent: Wednesday, December 02, 2009 4:52 PM
To: [3]<R-help at r-project.org>
Subject: [R] Problem with "Cannot compute correct p-values with ties"


   Dear All,
   1. why did the problem happen?
   2. How to solve it?

   --

Best wishes,
Zhijiang Wang
--------------------------------------------
PHD Student
Room 212, Science buliding,
The International WIC Institute,
College of Computer Science and Technology,
Beijing University of Technology,
Beijing, China.
______________________________________________
[4]R-help at r-project.org mailing list
[5]https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide [6]http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.




   --

Best wishes,
Zhijiang Wang
--------------------------------------------
PHD Student
Room 212, Science buliding,
The International WIC Institute,
College of Computer Science and Technology,
Beijing University of Technology,
Beijing, China.

References

   1. mailto:guohao.huang at gmail.com
   2. mailto:ivorytower at emails.bjut.edu.cn
   3. mailto:R-help at r-project.org
   4. mailto:R-help at r-project.org
   5. https://stat.ethz.ch/mailman/listinfo/r-help
   6. http://www.R-project.org/posting-guide.html

On Wed, 2009-12-02 at 16:52 +0800, Zhijiang Wang wrote:
> Dear All,
>    1. why did the problem happen?
>    2. How to solve it?
> 
>    --
> 
> Best wishes,
> Zhijiang Wang

Well... The algorithm for Mann-whitney test have problem with ties

To solve you can use jitter

a<-1:10
b<-1:10
wilcox.test(a,b)

	Wilcoxon rank sum test with continuity correction

data:  a and b 
W = 50, p-value = 1
alternative hypothesis: true location shift is not equal to 0 

Warning message:
In wilcox.test.default(a, b) : cannot compute exact p-value with ties

wilcox.test(a,jitter(b))

	Wilcoxon rank sum test

data:  a and jitter(b) 
W = 49, p-value = 0.9705
alternative hypothesis: true location shift is not equal to 0 

look ?jitter for more information

-- 
Bernardo Rangel Tura, M.D,MPH,Ph.D
National Institute of Cardiology
Brazil

Adding random noise to data in order to avoid a warning is like removing the
batteries from a smoke detector to silence it rather than investigating the what
is causing the alarm to go off.

If the function is giving a warning it is best to investigate why, it is
possible that you can ignore the warning (the burnt toast of smoke alarm
analogies) but it is best to convince yourself that it is ok.  It is also
possible in this case that another tool may be more appropriate, and
investigating the warning could help you find that tool.

-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Bernardo Rangel Tura
> Sent: Tuesday, December 22, 2009 1:16 AM
> To: ivorytower at emails.bjut.edu.cn
> Cc: R-help at r-project.org
> Subject: Re: [R] Problem with "Cannot compute correct p-values with
> ties"
> 
> On Wed, 2009-12-02 at 16:52 +0800, Zhijiang Wang wrote:
> > Dear All,
> >    1. why did the problem happen?
> >    2. How to solve it?
> >
> >    --
> >
> > Best wishes,
> > Zhijiang Wang
> 
> 
> Well... The algorithm for Mann-whitney test have problem with ties
> 
> To solve you can use jitter
> 
> a<-1:10
> b<-1:10
> wilcox.test(a,b)
> 
> 	Wilcoxon rank sum test with continuity correction
> 
> data:  a and b
> W = 50, p-value = 1
> alternative hypothesis: true location shift is not equal to 0
> 
> Warning message:
> In wilcox.test.default(a, b) : cannot compute exact p-value with ties
> 
> wilcox.test(a,jitter(b))
> 
> 	Wilcoxon rank sum test
> 
> data:  a and jitter(b)
> W = 49, p-value = 0.9705
> alternative hypothesis: true location shift is not equal to 0
> 
> look ?jitter for more information
> 
> --
> Bernardo Rangel Tura, M.D,MPH,Ph.D
> National Institute of Cardiology
> Brazil
> 
> ______________________________________________
> 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.