[R] Two factor ANOVA with lm()

Trenkler, Dietrich dtrenkler at nts6.oec.uni-osnabrueck.de
Mon Aug 23 13:39:28 CEST 2004

> -----Original Message-----
> From:	Prof Brian Ripley 
> Sent:	Monday, August 23, 2004 1:15 PM
> To:	Trenkler, Dietrich
> Subject:	Re: [R] Two factor ANOVA with lm()
> On Mon, 23 Aug 2004, Trenkler, Dietrich wrote:
> [...]
> > outset?  Or put another way:  Why is it that lm() uses the corner point
> > constraints by default?  Where can I find a documentation for this
> > behavior?
> In almost any piece of documentation on linear models in R, including the
> FAQ and `An Introduction to R', which says
>   The main reason for mentioning this is that R and S have different
>   defaults for unordered factors, S using Helmert contrasts.  So if you 
>   need to compare your results to those of a textbook or paper which used
>   S-PLUS, you will need to set
>     options(contrasts = c("contr.helmert", "contr.poly"))
>   This is a deliberate difference, as treatment contrasts (R's default)
>   are thought easier for newcomers to interpret.
> Now, what does the posting guide say about doing your homework?
	[Dietrich Trenkler]  I swear I didn't need it for a homework.
	I just overlooked the self-evident... (blush)

	Thank you.

	D. Trenkler

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