[R] Specifying xlevels in effects library

[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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