RE: p-values

[morphmet]

-------- Original Message --------
Subject: RE: p-values
Date: Fri, 28 Aug 2009 15:42:27 -0700 (PDT)
From: F. James Rohlf <ro...@life.bio.sunysb.edu>
Reply-To: ro...@life.bio.sunysb.edu
Organization: Stony Brook University
To: morphmet@morphometrics.org
References: <4a981aa5.3050...@morphometrics.org>

No rule of thumb but for such a simple calculation and sample sizes of 
just 10 and 10 you could use thousands of random permutations very quickly.

What you want to do each time is to take a random split of the 20
observations into two samples of size 10 and then compute your sample
statistic. Note that it does not have to be the t-statistic it can simply be
the difference between the two means. You then count how many times the
observed difference is equal to or larger than those obtained by the random
splits.

------------------------
F. James Rohlf, Distinguished Professor
Ecology & Evolution, Stony Brook University
www: http://life.bio.sunysb.edu/ee/rohlf


-----Original Message-----
From: morphmet [mailto:morphmet_modera...@morphometrics.org]
Sent: Friday, August 28, 2009 1:58 PM
To: morphmet
Subject: Re: p-values



-------- Original Message --------
Subject:        Re: p-values
Date:   Fri, 28 Aug 2009 10:28:31 -0700 (PDT)
From:   Samor Gandhi <samorgan...@yahoo.com>
To:     morphmet@morphometrics.org



Thank you very much! What I meant with the sample size was about the
following:

If we have two group, in each group 10 sample (i.e. n=20), the t.test
in
R can be used to get run the t-test (and/or Welch), and it gives the
t-value (t-statistic) and the theoretical p-value (from the t-
distribution)! What I am asking about is to get the t-value from the
t.test and run the test let say 100 times. What i wanted to know is
there an rule of thumb that tells how big n1 and n2 (n=n1+n2) should be
to run permutation for each time of these 100 times! So, would 8=10-2
be
good for each permutation! Because what I understand here (I am not a
mathematician) that one should compute first the t-value (i.e. using
t.test in R) and the compute 100 t-values due to permutation and then
see where does my first t-value lies and this would be the p-value! For
example if I have 4.332 as a t-value and this would be equal to the 8th
value in the permutations values (100 values), then my p-value would be
8/100=0.08! Is this correct?

Thank you in advance,
Samor

--- On *Fri, 8/28/09, morphmet
/<morphmet_modera...@morphometrics.org>/*
wrote:


     From: morphmet <morphmet_modera...@morphometrics.org>
     Subject: Re: p-values
     To: "morphmet" <morphmet@morphometrics.org>
     Date: Friday, August 28, 2009, 5:25 PM



     -------- Original Message --------
     Subject: Re: p-values
     Date: Fri, 28 Aug 2009 04:55:24 -0700 (PDT)
     From: Dennis E. Slice <dsl...@morphometrics.org
     </mc/compose?to=dsl...@morphometrics.org>>
     To: morphmet@morphometrics.org
     </mc/compose?to=morph...@morphometrics.org>
     References: <4a97c27b.2030...@morphometrics.org
     </mc/compose?to=4a97c27b.2030...@morphometrics.org>>

     Sokal and Rohlf. 1995. Biometry, 3rd ed.

         Section 9.4 - t-test
         Box 9.14 - Finding the sample size required and power for
ANOVA

     Rohlf and Sokal. 1995. Statistical Tables.

         Table B - critical values for t

     R Development Core Team (2005). R: A language and environment for
     statistical computing. R Foundation for Statistical Computing,
Vienna,
     Austria. ISBN 3-900051-07-0, URL http://www.R-project.org.

         Function: t.test()

     -dslice

     morphmet wrote:
      >
      >
      > -------- Original Message --------
      > Subject:     p-values
      > Date:     Thu, 27 Aug 2009 15:22:48 -0700 (PDT)
      > From:     Samor Gandhi <samorgan...@yahoo.com
     </mc/compose?to=samorgan...@yahoo.com>>
      > To:     morphmet_modera...@morphometrics.org
     </mc/compose?to=morphmet_modera...@morphometrics.org>
      >
      >
      >
      > Dear all,
      >
      > What is the best way to compute the p-vallue from the t-
statisic
      > (t-value)? And how big the subsample should be?
      >
      > Many thanks,
      > Samor
      >
      >
      >

     -- Dennis E. Slice
     Associate Professor
     Dept. of Scientific Computing
     Florida State University
     Dirac Science Library
     Tallahassee, FL 32306-4120
         -
     Guest Professor
     Department of Anthropology
     University of Vienna
         -
     Software worth having/learning/using...
     Linux (Operating System: Ubuntu, CentOS, openSUSE, etc.)
     OpenOffice (Office Suite: http://www.openoffice.org/)
     R package (Stats/Graphics environment: http://www.r-project.org/)
     Eclipse (Java/C++/etc IDE: http://www.eclipse.org/)
     ========================================================



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