Hi all,
I am trying to do a ordered probit regression using polr(), replicating a
result from SAS.
>polr(y ~ x, dat, method='probit')
suppose the model is y ~ x, where y is a factor with 3 levels and x is a
factor with 5 levels,
To get coefficients, SAS by default use the last level as reference, R by
default use the first level (correct me if I was wrong),
The result I got is a mixture, using first and last for different variables.
I tried relevel, reorder, contrasts, but no success. I found what really
matters is
>options(contrasts = c("contr.treatment", "contr.poly"))
or
>options(contrasts = c("contr.SAS", "contr.poly"))
so I guess I can set "contrasts= a list of contrasts" for each variables in
polr(), but I cannot successfuly set the contrasts, what is the syntax?
Is there a better way to do this?
Thank you very much,
Tian
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