R help - Apr 2007 - testing differences between slope differences with lme

hello

i have a mixed effect model which gives slope and intercept terms for 6 
groups  (diagnosis (3 levels) by risk group(2 levels)). the fixed part of 
the model is  --

brain volume ~ Diagnosis + Risk Group + (Risk Group * age : Diagnosis) - 1

thus allowing risk group age/slope terms to vary within diagnosis and 
omitting a nonsignificant diagnosis by risk group intercept (age was 
centered) interaction.

i am interested in whether differences in risk groups' developmental 
trajectories are different for different diagnoses.

the last three (of 10) fixed effect estimates are estimates for the 
age/slope differences between risk groups for the 3 diagnostic groups.

my question is this -- how do i test if the 3 differences are statistically 
different from each other?

i can do this in SAS but the answer in R is eluding me.


thanks in advance,
dede

Hi Dede,

I would construct vectors that represent the linear contrasts of
interest, and test them using the estimable() function of the gmodels
package.

Cheers,

Andrew

On Mon, Apr 09, 2007 at 07:18:54PM -0400, Dede Greenstein
wrote:
> hello
> 
> i have a mixed effect model which gives slope and intercept terms for 6 
> groups  (diagnosis (3 levels) by risk group(2 levels)). the fixed part of 
> the model is  --
> 
> brain volume ~ Diagnosis + Risk Group + (Risk Group * age : Diagnosis) - 1
> 
> thus allowing risk group age/slope terms to vary within diagnosis and 
> omitting a nonsignificant diagnosis by risk group intercept (age was 
> centered) interaction.
> 
> i am interested in whether differences in risk groups' developmental 
> trajectories are different for different diagnoses.
> 
> the last three (of 10) fixed effect estimates are estimates for the 
> age/slope differences between risk groups for the 3 diagnostic groups.
> 
> my question is this -- how do i test if the 3 differences are statistically
> different from each other?
> 
> i can do this in SAS but the answer in R is eluding me.
> 
> 
> thanks in advance,
> dede
> 
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-- 
Andrew Robinson  
Department of Mathematics and Statistics            Tel: +61-3-8344-9763
University of Melbourne, VIC 3010 Australia         Fax: +61-3-8344-4599
http://www.ms.unimelb.edu.au/~andrewpr
http://blogs.mbs.edu/fishing-in-the-bay/