Matrix of dummy variables from a factor

But note: There are (almost?) no situations in R where the dummy variables
coding is needed. The coding is (almost?) always handled properly by the
modeling functions themselves.

Question: Can someone provide a "straightforward" example where the dummy
variable coding **is** explicitly needed?

-- Bert Gunter
Genentech Non-Clinical Statistics
South San Francisco, CA
 
"The business of the statistician is to catalyze the scientific learning
process."  - George E. P. Box

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch 
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Liaw, Andy
Sent: Tuesday, December 06, 2005 12:30 PM
To: 'Charles H. Franklin'; r-help at stat.math.ethz.ch
Subject: Re: [R] Matrix of dummy variables from a factor

See ?model.matrix.

HTH,
Andy

From: Charles H. Franklin

What is a simple way to convert a factor into a matrix of 
dummy variables?

fm<-lm(y~f)

where f is a factor takes care of this in the estimation. 

I'd like to 

save the result of expanding f into a matrix for later use.

Thanks.

Charles

-- 


 Charles H. Franklin
 Professor, Political Science
 University of Wisconsin, Madison
 franklin at polisci.wisc.edu
 chfrankl at wisc.edu
 608-263-2022 (voice)
 608-265-2663 (fax)

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But note: There are (almost?) no situations in R where the dummy
variables coding is needed. The coding is (almost?) always handled
properly by the modeling functions themselves.

Question: Can someone provide a "straightforward" example where the
dummy variable coding **is** explicitly needed?

Perhaps not particularly straightforward, but if you are using
package 'mix' for multiple imputation where some variables are
categorical and some continuous, the functions

  ecm.mix

  dabipf.mix

both take a paremeter "design" which is a design matrix expressing
dependency of the continuous variables on the categoricals.

The R function 'model.matrix' is useful for obtaining this matrix.

Best wishes,
Ted.


--------------------------------------------------------------------
E-Mail: (Ted Harding) <Ted.Harding at nessie.mcc.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 06-Dec-05                                       Time: 22:39:44
------------------------------ XFMail ------------------------------

But note: There are (almost?) no situations in R where the dummy  
variables
coding is needed. The coding is (almost?) always handled properly  
by the
modeling functions themselves.

Question: Can someone provide a "straightforward" example where the  
dummy
variable coding **is** explicitly needed?

Bert's  question offers an opportunity for me to mention (again) my  
long standing wish
for someone to write a version of model.matrix that directly produced  
a matrix
in one of the common  sparse matrix formats.   This could be a good   
project for one of
you who like using ";" ?

Roger

url:    www.econ.uiuc.edu/~roger            Roger Koenker
email    rkoenker at uiuc.edu            Department of Economics
vox:     217-333-4558                University of Illinois
fax:       217-244-6678                Champaign, IL 61820