How to force R to use a specified factor level as reference in a regression?

Question

How can I tell R to use a certain level as reference if I use binary explanatory variables in a regression?It's just using some level by default. lm(x ~ y + as.factor(b)) with `b {0, 1, 2, 3, 4}`. Let's say I want to use 3 instead of the zero that is used by R.

Answer 1

To do this, you can relevel your factor variable. R will take the first level of the factor as the reference group (in your case, 0).

You can reassign b outside of the equation, i.e.:

b_fac <- factor(b, levels = c(3, 0, 1, 2, 4))

and then use the new factor variable in the model:

lm(x ~ y + b)

Or, you can assign the levels within the equation:

lm(x ~ y + factor(b, levels = c(3, 0, 1, 2, 4)))