[R] Code Verification

[Liaw, Andy]

> 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 at 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/
> --------------------------------------------------------------
> 
> pantd at unlv.nevada.edu writes:
>  > 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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