[R] subset of factors in a regression

[Philip A. Viton]

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<-state=="CA") and then including 
that). I know I can create all the necessary dummy variables by using 
the "outer" function on the factor and then renaming them 
appropriately; but is there a solution that's more direct, ie that 
doesn't involve a lot of new variables?

Thanks!


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Philip A. Viton
City Planning, Ohio State University
275 West Woodruff Avenue, Columbus OH 43210
viton.1 at osu.edu