[R] Specifying xlevels in effects library

[Liaw, Andy]

Here's an example:

> library(effects)
> age <- round(rnorm(100, mean=30, sd=5))
> sex <- factor(sample(c("M","F"), 100, replace=TRUE))
> y <- rnorm(100)
> fit <- lm(y ~ age + sex)
> eff <- effect("sex", fit, typical=function(...) 25)
> eff

 sex effect
sex
          F           M 
-0.23387685  0.07063834 

Now we can check if that looks right:

> predict(fit, newdata=data.frame(age=c(25, 25), sex=factor(c("M","F"))))
          1           2 
 0.07063834 -0.23387685 

HTH,
Andy



> From: Liaw, Andy
> 
> Prof. Fox will be able to give the definitive answer, but 
> from my reading of
> ?effect, xlevels refers to the values of the factor whose 
> effect you're
> interested in, not the ones being `marginalized'.  I believe 
> you need to
> play with the `typical' argument.
> 
> HTH,
> Andy
> 
> > From: David J. Netherway
> > 
> > library(effects)
> > mod <- lm(Measurement ~ Age + Sex, data=d)
> > e <-effect("Sex",mod)
> >  
> > The effect is evaluated at the mean age.
> >  
> >  > e
> > Sex effect
> > Sex
> >        F        M  
> > 43.33083 44.48531  
> >  >
> >  > e$model.matrix
> >    (Intercept)      Age SexM
> > 1            1 130.5859    0
> > 23           1 130.5859    1
> >  
> > To evaluate the effect at Age=120 I tried:
> > e <-effect("Sex",mod,xlevels=list(Age=c(120)))
> > but the effect was still evaluated at 130.5859.
> >  
> > Is this an incorrect usage of xlevels?
> > 
> > Thanks, David
> > 
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