[R] permutation test assumption?

João Fadista Joao.Fadista at agrsci.dk
Wed Apr 9 00:47:37 CEST 2008


But in that paper they refer to another (http://www.bepress.com/cgi/viewcontent.cgi?article=1053&context=ucbbiostat) where they say that "permutation distribution produces an asymptotically correct null distribution if (i) the sample sizes are equal...".
 
Best regards,
João

________________________________

De: Achim Zeileis [mailto:Achim.Zeileis at wu-wien.ac.at]
Enviada: qua 09-04-2008 0:26
Para: João Fadista
Cc: ONKELINX, Thierry; r-help at r-project.org
Assunto: Re: [R] permutation test assumption?



On Wed, 9 Apr 2008, João Fadista wrote:

> Dear Thierry,
>
> Thanks for the reply. But as you may read in the paper
> http://bioinformatics.oxfordjournals.org/cgi/content/abstract/22/18/2244
> when the sample sizes are not the same there may be an increase in the
> Type I error rate.

No, this is false and not what the reference above says. Please read more
carefully.

> Comments will be appreciated.

The basic assumption for permutation tests is exchangeability of the
observations (regardless of sample sizes). The reference above describes a
situation when this assumption is violated.
Z

> Best regards,
> João Fadista
>
>
> ________________________________
>
> De: ONKELINX, Thierry [mailto:Thierry.ONKELINX at inbo.be]
> Enviada: ter 08-04-2008 15:27
> Para: João Fadista; r-help at r-project.org
> Assunto: RE: [R] permutation test assumption?
>
>
>
> Dear João,
>
> You can do permutation tests on an unbalanced design.
>
> HTH,
>
> Thierry
>
>
> ----------------------------------------------------------------------------
> ir. Thierry Onkelinx
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> -----Oorspronkelijk bericht-----
> Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] Namens João Fadista
> Verzonden: dinsdag 8 april 2008 15:18
> Aan: r-help at r-project.org
> Onderwerp: [R] permutation test assumption?
>
> Dear all,
>
> Can I do a permutation test if the number of individuals in one group is much bigger than in the other group? I searched the literature but I didin´t find any assumption that refers to this subject for permutation tests.
>
>
> Best regards
>
> João Fadista
> Ph.d. student
>
>
>
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