Re: ANOVA with selected comparisons?

Peter Frank wrote:
> "Anon." wrote:
>
>
> > Peter Frank wrote:
> >
> > > Hello,
> > >
> > > I got a question regarding multiple comparison analysis. I have six
> > > samples and I am only interested in the comparisons of one group of
> > > three samples to the other group of three samples, which makes 9
> > > comparisons. Doing an ordinary ANOVA with the SNK post hoc test gives
> > > me more comparisons than that. Are the results still valid or do I
> > > have to make any corrections for being only interested in comparing
> > > one group of samples to the other group of samples and not any sample
> > > to any other sample?
> >
> > Why don't you use a nested ANOVA?  Nest Samples within Groups, and test 
> > the Groups effect.
>
>
> Hi Bob,
>
> How do I do a nested ANOVA? BTW, each of the six samples consists of
> three measurements and thus has its own variance. Is there any common
> statistical software package that can do a nested ANOVA?
Any decent stats software - R, S-PLUS, SAS, and Genstat all do it, and 
I'll be shocked if SPSS doesn't.

The model is that each sample has a mean (of the three measurements), 
and each group has a common mean (of the sample means).  The ANOVA tests 
whether the group means are the same, and then whether the sample means 
are the same.  Depending on what package you're using, the way the model 
is written is different.

Any decent stats textbook (including those for non-specialists) should 
describe nested ANOVA, so you should pick your favourite (for example 
Grafen & Hails' book seems to have got good reviews, it's aimed at 
biologists but that shouldn't put you off).

Bob

--
Bob O'Hara

Rolf Nevanlinna Institute
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