# Re: [R] Code Verification

From: Liaw, Andy <andy_liaw_at_merck.com>
Date: Wed 20 Jul 2005 - 23:52:55 EST

> From: Christoph Buser
>
> Hi
>
> "t.test" assumes that your data within each group has a normal
> distribution. This is not the case in your example.

Eh? What happen to the CLT?

Andy

> I would recommend you a non parametric test like "wilcox.test" if
> you want to compare the mean of two samples that are not normal
> distributed.
> see ?wilcox.test
>
> Be careful. Your example produces two gamma distributed samples
> with rate = 10, not scale = 10.
> rate = 1/scale.
> If you want to use scale, you need to specify this argument
> x<-rgamma(40, 2.5, scale = 10)
> see ?rgamma
>
> I do not see the interpretation of your result. Since you do
> know the distribution and the parameters of your sample, you
> know the true means and that they are different. It is only a
> question of the sample size and the power of your test, if this
> difference is detected.
> Is that something you are investigating? Maybe a power
> calculation or something similar.
>
> Regards,
>
> Christoph Buser
>
> --------------------------------------------------------------
> Christoph Buser <buser@stat.math.ethz.ch>
> Seminar fuer Statistik, LEO C13
> ETH (Federal Inst. Technology) 8092 Zurich SWITZERLAND
> phone: x-41-44-632-4673 fax: 632-1228
> http://stat.ethz.ch/~buser/
> --------------------------------------------------------------
>
> > Hi R Users
> > I have a code which I am running for my thesis work. Just
> want to make sure that
> > its ok. Its a t test I am conducting between two gamma
> distributions with
> > different shape parameters.
> >
> > the code looks like:
> >
> > sink("a1.txt");
> >
> > for (i in 1:1000)
> > {
> > x<-rgamma(40, 2.5, 10) # n = 40, shape = 2.5, Scale = 10
> > y<-rgamma(40, 2.8, 10) # n = 40, shape = 2.8, Scale = 10
> > z<-t.test(x, y)
> > print(z)
> > }
> >
> >
> > I will appreciate it if someone could tell me if its alrite or not.
> >
> > thanks
> >
> > -dev
> >
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