R help - Jun 2006 - help with contrasts.

Hello list,

I would like to know how can I use contrats in statistical analysis. I
have a simple model, "one-way analysis of variance" "or
two-way..."
(balanced case).

I've already used the command aov (model) to get  the analysis of
variance, but i know that my treatment could be organized in groups,
so i would like to test these groups.

Example: I have 4 treatments t1, t2, t3, t4 and i would like to test
(1, 1, -1, -1) and (1, -1, 0, 0) and (0, 0, 1, -1).

thanks for the attention.

Rodrigo Gazaffi

Dear Rodrigo,

One way to do what you want is by

contrasts(f) <- matrix(
  c(1, 1, -1, -1,   1, -1, 0, 0,   0, 0, 1, -1), 
  ncol=3)

See ?contrasts for details.

I hope this helps,
 John

On Sun, 25 Jun 2006 11:48:57 -0300
 Rodrigo Gazaffi <rgazaffi at carpa.ciagri.usp.br>
wrote:
> Hello list,
> 
> I would like to know how can I use contrats in statistical analysis.
> I
> have a simple model, "one-way analysis of variance" "or
two-way..."
> (balanced case).
> 
> I've already used the command aov (model) to get  the analysis of
> variance, but i know that my treatment could be organized in groups,
> so i would like to test these groups.
> 
> Example: I have 4 treatments t1, t2, t3, t4 and i would like to test
> (1, 1, -1, -1) and (1, -1, 0, 0) and (0, 0, 1, -1).
> 
> thanks for the attention.
> 
> Rodrigo Gazaffi
> 
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--------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada
http://socserv.mcmaster.ca/jfox/