Suppose I want to run a regression using lm and a factor as a right hand side variable. What is the best way to choose which level in the factor is the base category (the one that is excluded to avoid multicollinearity). Note that I am not interested in excluding the intercept because I have many factors.
I would also like a formula-based solution, not one that acts on the data.frame directly, although if you think you have a really good solution for that, please post it as well.
My solution is:
base_cat <- function(x) c(x,1:(x-1),(x+1):100)
a_reg <- lm(y ~ x1 + x2 + factor(x3, levels=base_cat(30)) #suppose that x3 has draws from the integers 1 to 100.
The left out category by lm is the first level in the factor so this just reorders the levels so that the one specified in base_cat() is the first one, and puts the rest after.
Any other ideas?
The function
releveldoes precisely this. You pass it an unordered factor and the name of the reference level and it returns a factor with that level as the first one.