In addition to David's great answer also read

?contr.SAS
?contr.treatment
?relevel

and also sections 4 and 11.1.1 in An Introduction to R.


On Fri, May 3, 2013 at 9:47 PM, David Winsemius <dwinsemius@comcast.net>wrote:

>
> On May 3, 2013, at 3:32 PM, Iuri Gavronski wrote:
>
> > Hi,
> > I would like to know the criteria by which R removes a factor in linear
> > models. For example, I have a four level factor, and R creates 3 dummies
> to
> > estimate coefficients. Which level is chosen? Can I chance it?
>
> The default order is alphabetical. Lowest lexical sorted item is the
> reference level.
>
> Changing levels is possible:
>
> ?levels
> ?factor
>
> --
> David Winsemius
> Alameda, CA, USA
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>



-- 
Gregory (Greg) L. Snow Ph.D.
538280@gmail.com

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