[R] Linear Contrasts in GLM - Query

[ONKELINX, Thierry]

Have a look at glht() from the multcomp package. That will allow you to
specify the contrasts for posthoc tests.
A more direct approach is the create the required dummy variables
yourself.

HTH,

Thierry


------------------------------------------------------------------------
----
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium
tel. + 32 54/436 185
Thierry.Onkelinx at inbo.be
www.inbo.be

To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey

-----Oorspronkelijk bericht-----
Van: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
Namens Leo Guelman
Verzonden: vrijdag 4 september 2009 13:14
Aan: r-help
Onderwerp: [R] Linear Contrasts in GLM - Query

Hi,

Is there a way I can specify linear contrasts in glm? I'm looking for
something equivalent to SAS' contrast statement.

I'd like to do the following, suppose I have a categorical input with 4
levels (a,b,c,d), I'd like to test something like: (i)  a+b=c+d, (ii)
a=b,
(iii) a=b+d, etc...

Thanks in advance for your help!

Leo.

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