[R] Help - lm, glm, aov results inconsistent with other stati stical package
andy_liaw at merck.com
Wed Mar 1 02:14:57 CET 2006
1. You have levels(A) as "2" and "4", yet you showed equations for A=0 and
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.
From: Ben Ridenhour
> 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
>  "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!
> 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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