R help - Jan 2008 - covariate in a glm

Hello mailing list!

I would like to know, how I can introduce a covariate in a glm, I've two
factors and a covariate.

Thank you very much!

_______________________________________________________________________________________________

Michelangelo La Spina 

Equipo de Protección de cultivos - Control Biológico 
Departamento de Biotecnología y Protección de Cultivos 
Instituto Murciano de Investigación y Desarrollo Agrario y Alimentario (IMIDA) 
C/ Mayor s/n
30150 La Alberca (Murcia) - ESPAÑA 

teléfono: +34 968 362 788 
fax: +34 968 366 792 
móvil: +34 618 451 079 

web: http://www.imida.es/equipos/eq_prot_cultivos_comp.html
e-mail: michelangelo.la@carm.es


	[[alternative HTML version deleted]]

On Tue, 2008-01-15 at 11:15 +0100, LA SPINA, MICHELANGELO
wrote:
> Hello mailing list!
> 
> I would like to know, how I can introduce a covariate in a glm, I've
> two factors and a covariate.
> 
> Thank you very much!
> 
> ____________________________________________________________________
> Michelangelo La Spina 
Hi,

I'm not clear on the distinction between a factor and a covariate (I my
way of thinking, a covariate is a variable that might predict an outcome
or response variable). If you want to use the "covariate" as the
response variable in a GLM then something like the following may be
appropriate

## dummy data
fac1 <- sample(gl(4, 25))
fac2 <- sample(gl(2, 50))
covar <- rnorm(100)
my.dat <- data.frame(fac1 = fac1, fac2 = fac2, covar = covar)
mod1 <- glm(covar ~ fac1 + fac2, data = my.dat)
mod1
summary(mod1)

# or if you already have a response variable and you want to include the
covariate on the RHS of the formula, then one simple adds it in the same
manner as we added the factors to the RHS above

## new dummy response
my.dat$resp <- rt(100, df = 3)
## new model
mod2 <- glm(resp ~ fac1 + fac2 + covar, data = my.dat)
mod2
summary(mod2)

Does this answer your question?

G
-- 
%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%
 Dr. Gavin Simpson             [t] +44 (0)20 7679 0522
 ECRC, UCL Geography,          [f] +44 (0)20 7679 0565
 Pearson Building,             [e] gavin.simpsonATNOSPAMucl.ac.uk
 Gower Street, London          [w] http://www.ucl.ac.uk/~ucfagls/
 UK. WC1E 6BT.                 [w] http://www.freshwaters.org.uk
%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%