[R] Dummies in R
tlumley at u.washington.edu
Thu Apr 29 16:08:43 CEST 2004
On Thu, 29 Apr 2004, [iso-8859-1] Susana Bird wrote:
> Dear all,
> my problem is following. I know Stata, but currently I have to use R.
> Could You please help in finding the analogy to R.
> (1) creating of City-Dummy.
> (2) Create a Time-Dummy-Variable
Andy Liaw has described how to do this, but you probably don't need to.
In regression models in R, factor variables are automatically expanded to
a suitable design matrix, by default a set of indicator (`dummy')
variables for each category except the first.
regModel <- lm( outcome~factor(city)*factor(time))
will give you indicator variables for each city and time and for the
otherRegModel <- lm(outcome~factor(city)+factor(time))
is the model without interactions.
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