Margins Package error using quadratic and interaction terms

Question

I have code which uses the margins command in Stata and I am trying to replicate it in R using the "margins" package found here and on cran.

I keep getting the error:

marg1<-margins(reg2)  

Error in names(classes) <- clean_terms(names(classes)) : 'names' attribute [18] must be the same length as the vector [16]"

A minimum reproducible example is show below:

install.packages(margins)
library(margins)
mod1 <- lm(log(mpg) ~ vs + cyl + hp + vs*hp + I(vs*hp*hp) + wt + I(hp*hp), data = mtcars)
(marg1 <- margins(mod1))
summary(marg1)

I need vs to be a dummy variable interacted with both a quadratic term and a normal interaction.

Does anyone know what I am doing wrong or if there is a way around this?

Answer 1

Your model specification is a bit confusing. For example, vs*hp introduces 3 variables: i) vs, ii) hp and iii) interaction vs and hp. As a result, hp appears twice in the formula you provided. You can simplify massively! Try this for example (I think it is what you want):

mtcars$hp2 = mtcars$hp^2
mod1 <- lm(log(mpg) ~ cyl + wt + vs*hp + vs*hp2, data = mtcars)
summary(mod1) # With this you can check that the model you specified is what you want
(marg1 <- margins(mod1)) # The error disappeared. 
summary(marg1)

In general, I would recommend you to avoid I() in formula specifications, as it often gives rise to such errors when not treated with enough care (though sometimes one cannot avoid it). Good luck!