[Rd] matching predictors and dummies
Charles C. Berry
cberry at tajo.ucsd.edu
Fri Jul 11 18:26:33 CEST 2008
On Fri, 11 Jul 2008, Jeroen Ooms wrote:
> I am trying to make a little web interface for the lm() function. It
> calculates both anova F-tests and parameters and returns it in a nice table.
> However, I have a problem with matching the Anova predictors with the
> regression coefficients: For numeric predictors there is no problem: the
> coefficients have the same names as the predictors. However, when a factor
> IV is specified, lm() automatically converts this factor to dummy variables,
> which (of course) have different names than the orriginal predictor. The lm
> model that is returned contains a seperate parameter for every dummy
> Then when you use anova(lm.model) the function seems to know which of the
> parameters are dummies of one and the same factor, and takes these together
> in the anova-test. The anova() function returns the variance explained by
> the orriginal factor, which are all dummies. It does not show the seperate
> dummy variables anymore. Of course, this is exactly what you want in an
> analysis of variance.
> My question is: where in the lm or glm object is stored which of the
> parameters are dummies of the same factor? The only thing i could think of
> was using lm.model$xlevels, however manipulating these names in the lm-model
> did not confuse anova() at all, so I guess there is a better way.
example( terms.object )
or something like
terms( lm( Ozone ~ Temp + factor(Month), airquality ) )
> An additional question is if it is possible to specify the names of the
> dummy variables that lm/glm creates when a factor is specified as IV?
I'm guessing a custom contrast function would do this. Have a look at
page( contr.treatment, 'print' )
Or just hack the names attribute of the relevant pieces in the object
returned by lm/glm.
You do know to use str(), right?
> View this message in context: http://www.nabble.com/matching-predictors-and-dummies-tp18405023p18405023.html
> Sent from the R devel mailing list archive at Nabble.com.
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Charles C. Berry (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901
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