[R] How to specify a quadratic term in the contrast function

[Ben Bolker]

   Looking at the 'contrasts' function, it seems to be imported from 
'rms' and to take only 'rms'-type fits (which are different from base-R 
[g]lm() fits) as input.

   I'm not sure exactly what contrasts you're trying to generate, but I 
would probably use the emmeans package for the task, since it handles a 
wide range of model types.

  Alternately, you could use one of the modeling functions from rms, 
which should work with rms::contrast(), although I ran into trouble for 
some reason:

fit1 <- ols(y ~ x + I(x^2), data = mydata)

 > Error in if (!length(fname) || !any(fname == zname)) { :
   missing value where TRUE/FALSE needed

I guess rms handles I()-protected terms differently? (The model with 
just y~x didn't throw an error ...)

On 2025-02-18 12:00 p.m., Sorkin, John wrote:
> I am using the contrast package to produce contrasts from a glm model that contains a quadratic term:
> fit0 <- glm(y ~ x + I(x*x),data=mydata)
> I would like to know how to specify the quadratic term, I(x*x), in the contrast function.
> When I try to use
> contrast(fit0,list(x=4))
> I receive an error message;
> #Error in generateData(fit = list(coefficients = c(`(Intercept)` = 0.5262645536229,  :not enough factors
> When I try to use
> contrast(fit0,list(x=4,I(x*x)=16))
> I receive an error message:
> #Error: unexpected '=' in "contrast(fit0,list(x=4,I(x*x)="
> Other variants of the call to contrast including
> contrast(fit0,list(x=4,"x*x"=16)) and
> contrast(fit0,list(x=4,"I(x*x}"=16))
> produce similar error messsages.
> 
> Please see my code below.
> 
> if (!require(contrast)) install.packages("contrast")
> library(contrast)
> help(contrast)
> 
> x <- 1:100
> y <- x + x^2 + rnorm(1)
> mydata <- data.frame(x=x,y=y)
> 
> fit0 <- glm(y ~ x + I(x*x),data=mydata)
> summary(fit0)
> 
> contrast(fit0,list(x=4))
> #Error in generateData(fit = list(coefficients = c(`(Intercept)` = #0.5262645536229,  :  not enough factors
> 
> contrast(fit0,list(x=4,I(x*x)=16))
> #Error: unexpected '=' in "contrast(fit0,list(x=4,I(x*x)="
> 
> Thank you,
> John
> 
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-- 
Dr. Benjamin Bolker
Professor, Mathematics & Statistics and Biology, McMaster University
Director, School of Computational Science and Engineering
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