[R] Is the Intercept Term always in First Position?

[Viechtbauer Wolfgang (STAT)]

Dear All,

I have a question about formulas and model.matrix(). If one specifies a model via a formula, the corresponding design matrix can be obtained with the model.matrix() function. For example:

x1 <- c(1,4,2,3,5)
x2 <- c(1,1,2,2,2)
myformula <- ~ x1 + factor(x2)
model.matrix(myformula)

My question is: If an intercept term is in the model (like in the example above), is it always the first column in resulting design matrix?

For example, if I add the intercept explicitly as the last term in the formula, it still ends up in the first column:

myformula <- ~ x1 + factor(x2) + 1
model.matrix(myformula)

So, is this always true or is it in principle possible that the intercept column ends up somewhere else?

Best,

--
Wolfgang Viechtbauer                        http://www.wvbauer.com/
Department of Methodology and Statistics    Tel: +31 (0)43 388-2277
School for Public Health and Primary Care   Office Location:
Maastricht University, P.O. Box 616         Room B2.01 (second floor)
6200 MD Maastricht, The Netherlands         Debyeplein 1 (Randwyck)