On Mar 28, 2010, at 1:15 PM, Ravi Kulkarni wrote:
Hello,
I am trying to do a logistic regression and have one predictor
variable
(x) that is ratio and two predictor variables (y and z) that are
categorical. These have three levels each which I have called "High",
"Medium" and "Low".
My question: do I need to use a numerical coding scheme for the
categorical variables as required by some statistical software
packages,
with some sort of numeric dummy-variable coding?
No. If you have constructed those variables as factors, the regression
functions in R will interpret them correctly, i.e. as though the
dummies were in there. If you have not constructed them as factors,
you should do so now.
?factor
?levels
I am using:
glm(binvar~x+y+z, family=binomial(link="logit"))
Thanks,
Ravi Kulkarni
--
David Winsemius, MD
West Hartford, CT
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