R help - Oct 2007 - Question about aov

Hello R gurus,
   
  I am a beginner with R.  I am doing an ANCOVA analysis using 'aov,'
and need some help understanding how 'aov' works.  I have a dataset
(taken from http://faculty.vassar.edu/lowry/ch17pt2.html) looking at hypnotic
induction.  The variable 'X' is a measure of how susceptible the subject
is to being hypnotized, the variable 'Y' is how well the subject was
hypnotized in the experiment, and the variable 'Method' is the method of
hypnosis used in the experiment.
   
  If I use the following code, I get the correct analysis (F = 16):
   
  ANCOVA=aov(lm(Y~X+Method,data=hypnotic.induction))
summary(ANCOVA);
   
  But if I switch the order of independent variables to: 
   
  ANCOVA=aov(lm(Y~Method+X,data=hypnotic.induction))
summary(ANCOVA)
   
  I get a very different result (F = 0.8).
   
  I assume it has something to do with the order in which the variables are
entered into the regression? In 'aov' is there something
'special' about the first independent variable entered into the formula?
It would be greatly appreciated if anyone could shed some light on these
results?
   
  Thanks!
   
  Alex Keedy

       
---------------------------------

	[[alternative HTML version deleted]]

Dear Alex,

First, the call to lm() is superfluous; you'd get the same output from
summary(aov(Y~X+Method,data=hypnotic.induction)). Second, it's not common to
use aov() in this manner, since the function is really intended for balanced
ANOVAs. More common would be something like

model <- lm(Y~X+Method,data=hypnotic.induction)
summary(model)
anova(model)

This would give you both the regression coefficients for the model and the
ANOVA table.

Finally, aov() and anova() produce "sequential" sums of squares, which
is
why the order of the terms matters. Thus the first ordering gives you the
sum of squares for X "ignoring" Method and for Method
"after" X; the second
gives you the sum of squares for Method "ignoring" X and for X
"after"
Method. What one typically wants is the sum of squares for each term after
the other(s). A convenient way to get this is via the Anova() function in
the car package.

I hope this helps,
 John

--------------------------------
John Fox, Professor
Department of Sociology
McMaster University
Hamilton, Ontario
Canada L8S 4M4
905-525-9140x23604
http://socserv.mcmaster.ca/jfox 
-------------------------------- 
> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Alexander Keedy
> Sent: Sunday, October 07, 2007 2:37 PM
> To: r-help at r-project.org
> Subject: [R] Question about aov
> 
> Hello R gurus,
>    
>   I am a beginner with R.  I am doing an ANCOVA analysis 
> using 'aov,' and need some help understanding how 'aov' 
> works.  I have a dataset (taken from 
> http://faculty.vassar.edu/lowry/ch17pt2.html) looking at 
> hypnotic induction.  The variable 'X' is a measure of how 
> susceptible the subject is to being hypnotized, the variable 
> 'Y' is how well the subject was hypnotized in the experiment, 
> and the variable 'Method' is the method of hypnosis used in 
> the experiment.
>    
>   If I use the following code, I get the correct analysis (F = 16):
>    
>   ANCOVA=aov(lm(Y~X+Method,data=hypnotic.induction))
> summary(ANCOVA);
>    
>   But if I switch the order of independent variables to: 
>    
>   ANCOVA=aov(lm(Y~Method+X,data=hypnotic.induction))
> summary(ANCOVA)
>    
>   I get a very different result (F = 0.8).
>    
>   I assume it has something to do with the order in which the 
> variables are entered into the regression? In 'aov' is there 
> something 'special' about the first independent variable 
> entered into the formula?  It would be greatly appreciated if 
> anyone could shed some light on these results?
>    
>   Thanks!
>    
>   Alex Keedy
> 
>        
> ---------------------------------
> 
> 	[[alternative HTML version deleted]]
> 
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