subset of factors in a regression

Ben Bolker · 2013-07-02T04:39:59Z

Philip A. Viton osu.edu> writes: > suppose "state" is a variable in a dataframe containing abbreviations > of the US states, as a factor. What I'd like to do is to include > dummy variables for a few of the states, (say, CA and MA) among the > independent variables in my regression formula. (This would be the > equivalent of, creating, eg, ca that). I know I can create all the necessary dummy variables by using > the "outer" function on the

Ben Bolker

Mon, Jul 1, 2013 9:39 PM

Philip A. Viton <viton.1 <at> osu.edu> writes:

You could use model.matrix(~state-1) and select the columns
you want, e.g.

state <- state.abb; m <- model.matrix(~state-1)
m[,colnames(m) %in% c("stateCA","stateMA")]

 -- but this will actually create a bunch of vectors you
want before throwing them away.

more compactly:

m <- sapply(cstates,"==",state)
storage.mode(m) <- "numeric"
## or m[] <- as.numeric(m)

subset of factors in a regression

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