[R] Help - lm, glm, aov results inconsistent with other stati stical package

[Liaw, Andy]

1. You have levels(A) as "2" and "4", yet you showed equations for A=0 and
A=1?

2. y = A + X + A*X means you're allowing the different groups of A to have
different slopes.  Probably not what you intended.

3. It's probably best to provide a small sample of the data (and R code) so
we know how you got what you got.

Andy

From: Ben Ridenhour
> 
> Hello,
>  
>  I 'm sure there must a be a simple explanation for what I'm 
> doing wrong but I am stumped.  I am a novice R user and this 
> has shaken my confidence in what I'm doing! I am trying to 
> run a simple ANCOVA using the model y~A*x, where y and x are 
> continuous and A has two levels.  Everything seems fine until 
> I compare the output with what I get from another statistical 
> package (JMP).  JMP has the model y=A+x+A*x (this should be 
> the same as what I specified to R, correct?).  In comparison 
> I get the line equations
>  
>  y = 7072.09-1024.94 x (for A=0) and
>  y = 7875.58-970.088 x (for A=1)
>  
>  from JMP.  And from R I get
>  
>  y = 6276.7-1259.8 x (for A=0) and
>  y = 7867.5-1150.1 x (for A=1).
>  
>  Obviously, these aren't even close to the same answer.  I've 
> tried this using glm(), lm(), and aov() and as expected they 
> all give the same answer.  If I do
>  
>  >levels(A)
>  [1] "2" "4"
>  
>  which are the two levels of A.  Why can't I get the same 
> answer from JMP as in R?  This is very disturbing to me!
>  
>  Thanks,
>  Ben
>  
>  
> -------------------------------
> Benjamin Ridenhour
> School of Biological Sciences
> Washigton State University
> P.O. Box 644236
> Pullman, WA 99164-4236
> Phone (509)335-7218
> -------------------------------- 
> "Nothing in biology makes sense except in the light of evolution."
> -T. Dobzhansky
> 
> 
> 
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