[R] Select some, but not all, variables stepwise

Francisco Mora Ardila fmora at oikos.unam.mx
Fri Nov 4 14:20:25 CET 2011

Hi Andre

I don´t know if it will work, but I´ve tried the MuMIn package, were you can evaluate 
all possible models (usin for example AIC) at one time. Maybe you can focus on comparing 
those models which retain the explanators you want.

Best wishes

On Fri, 4 Nov 2011 13:06:09 +0000, Andre Easom wrote
> Hi,
> I would like to fit a linear model where some but not all explanators are 
> chosen stepwise - ie I definitely want to include some terms, but others only 
> if they are deemed significant (by AIC or whatever other approach is available)
> .  For example if I wanted to definitely include x1 and x2, but only include 
> z1 and z2 if they are significant, something like this:
> df <- data.frame(y=c(4,2,6,7,3,9,5,7,6,2), x1=c(2,3,4,0,5,8,8,1,1,2), x2=c(0,0,
> 0,0,1,1,0,0,0,1), z1=c(0,1,0,0,0,1,1,0,1,1), z2=c(1,1,1,0,0,1,1,1,1,0)) model 
> <- lm(y  ~ x1 + x2 + stepwise(z1 + z2))
> Any help would be appreciated.
> Cheers,
> Andre
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Francisco Mora Ardila
Laboratorio de Biodiversidad y Funcionamiento del Ecosistema
Centro de Investigaciones en Ecosistemas
UNAM-Campus Morelia
Tel 3222777 ext. 42621
Morelia , MIchoacán, México.

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