T Mu <muster <at> gmail.com> writes:
>
> 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),
Yes.
> I tried relevel, reorder, contrasts, but no success. I found what really
> matters is
I am sure those can help but you need to be carefull to "reorder" levels
that the order is the same in SAS and R.
> >options(contrasts = c("contr.treatment", "contr.poly"))
>
> or
>
> >options(contrasts = c("contr.SAS", "contr.poly"))
You can also set contrasts directly to factors via
contrasts(x) <- contr.SAS
where x is your factor. You can also set different contrasts to
different factors.
Gregor
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