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matching predictors and dummies

4 messages · Jeroen Ooms, Charles C. Berry, Mark Difford

Original
Jeroen Ooms
# 
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
variable.

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.

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?
Charles C. Berry
#
On Fri, 11 Jul 2008, Jeroen Ooms wrote:

            

      
      
      
    

        
See
 	?terms
 	?terms.object

and run

 	example( terms.object )

or something like

 	terms( lm( Ozone ~ Temp + factor(Month), airquality ) )

            

      
      
      
    

        
I'm guessing a custom contrast function would do this. Have a look at

 	?contrasts
 	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?


HTH,

Chuck

            

      
      
      
    

        

      
      
    

        
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

  
  


  


      

    

    

      
      

        


  

        

      Jeroen Ooms
    

    

      
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Charles C. Berry wrote:

            

      
      
      
    

        
I am sorry but i cannot figure out how i can find out which coefficients
belong to which predictors using the model$terms. If i do
attributes(model$terms) i get a nice list which contains the orriginal
factors and some more information, and from model$xlevels i can see the
levels of these factors. However, it does not say anywhere which of the
model$coefficients belong to which factor. The only thing i can imagine is
assuming that the dummy names are simply the concatenation of the factor
name and the level name, which seems to be the default behavior. But
preferebly i would not want to make my program rely on this assumption. How
could i use the model$terms to extract which coefficients belong to which
factor, the way anova() does it?

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Mark Difford
    

    

      
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Hi Jeroen,

            

      
      
      
    

        
There may be a simpler ("canned") way to do it, but why don't you debug
anova.lm to see how it does it?

##
methods("anova")
anova.lm

HTH, Mark.

        
Jeroen Ooms wrote: