[R] Help with contrasts

[Guelman, Leo]

Or alternatively (though very similar to Peter's idea) you can do

ci <- contrasts
formals(ci)$contrasts <- FALSE
dd <- data.frame(a = gl(3,4), b = gl(4,1,12))
mm <- model.matrix(~ a + b, dd, contrasts = list(a=ci(dd$a),
b=ci(dd$b))) 

Best,
Leo.

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of peter dalgaard
Sent: 2011, May, 11 7:31 AM
To: Lars Bishop
Cc: R-help at r-project.org Help
Subject: Re: [R] Help with contrasts


On May 11, 2011, at 12:51 , Lars Bishop wrote:

> Hi,
> 
> I need to build a function to generate one column for each level of a 
> factor in the model matrix created on an arbitrary formula (instead of

> using the available contrasts options such as contr.treatment,
contr.SAS, etc).
> 
> My approach to this was first to use the built-in function for 
> contr.treatment but changing the default value of the contrasts 
> argument to FALSE (I named this function "contr.identity" and it shown

> at the bottom of the email for reference).
> 
> So this function works fine,
> 
>> contr.identity(4)
> 
>  1 2 3 4
> 1 1 0 0 0
> 2 0 1 0 0
> 3 0 0 1 0
> 4 0 0 0 1
> 
>> contr.treatment(4)
>  2 3 4
> 1 0 0 0
> 2 1 0 0
> 3 0 1 0
> 4 0 0 1
> 
> However, when I try to create a model matrix using contr.identity 
> specified in options(), it actually uses the contr.treatment option. 
> Why is that? Any hint on how can I do this?


It's not actually using contr.treatment, it's just calling
contr.identity with contrasts=TRUE...

I don't think there's a painless way to avoid this. The closest I can
think of is

cI <- contr.treatment
formals(cI)$contrasts <- FALSE
dd <- data.frame(a = gl(3,4), b = gl(4,1,12)) model.matrix(~ C(a,cI,3) +
C(b,cI,4), dd)



--
Peter Dalgaard
Center for Statistics, Copenhagen Business School Solbjerg Plads 3, 2000
Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com

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