On Thu, Mar 21, 2013 at 8:59 PM, Laura M. Tully <tully.laura@googlemail.com> wrote:
oops sorry! (I also miscalculated the # of regressors - there's actually  12 (not 10 as previously noted). Here is the list of column labels: 
Grp1male  Grp1female Grp2male Grp2female Grp1maleVar1 Grp1femaleVar1 Grp1maleVar2 Grp1femaleVar2 Grp2maleVar1 Grp2femaleVar1 Grp2maleVar2 Grp2femaleVar2

And what I think is actually an F test looking for group x var 1 interaction OR group x variable 2 interaction whilst accounting for gender. 
.5 .5 -.5 -.5 0 0 0 0 0 0
0 0 0 0 0 0 .5 .5 -.5 -.5

The Contrast for group*var1 would be: 0 0 0 0 .5 .5 0 0 -.5 -.5 0 0 
The Contrast for group*var2 would be: 0 0 0 0 0 0 .5 .5 0 0 -.5 -.5

 

But what I actually WANT to test is a  multiple regression style model - i.e. if I put var 1 AND 2 into the model together do they explain more variance than either variable alone, AND does this vary by group (is this even a sensible contrast to make?). Which I *think* would look something like this... 

0 0 0 0 .125 .125 .125 .125 .125 .125 .125 .125

People generally don't ask that question.  The answer is when you add more variables, you will explain more variance. Tests about overall model fits are generally assessed with the AIC, BIC, etc. metrics. I'm not sure if there is anyway in regression to say that the amount of variance explained is different by group unless you run 2 separate models. If you think this might be a valid question, I'd consult a statistician - which I am not.
 

Laura. 



On Thu, Mar 21, 2013 at 5:51 PM, MCLAREN, Donald <mclaren.donald@gmail.com> wrote:
Please include the list of the column labels.

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Postdoctoral Research Fellow, GRECC, Bedford VA
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Thu, Mar 21, 2013 at 6:43 PM, Laura M. Tully <tully.laura@googlemail.com> wrote:
hi Experts, 

I'm struggling to conceptualize the appropriate contrasts for my cortical thickness analysis. I have four classes [two groups; two levels (patients,controls; male,female) and two behavioral variables. I want to see if together the two variables account significant proportion of the variance in y (thickness) and if this differs by group whilst regressing out gender. - i.e. if I enter both behavioral variables into the model does it account for more variance than either variable on their own (after controlling for gender)? What I have is this: 

.5 .5 -.5 -.5 0 0 0 0
0 0 0 0 .5 .5 -.5 -.5

Does this look right? 

Thanks! 

Laura. 




--
--
Laura M. Tully, MA
Social Neuroscience & Psychopathology, Harvard University
Center for the Assessment and Prevention of Prodromal States, UCLA Semel Institute of Neuroscience
ltully@mednet.ucla.edu 
ltully@fas.harvard.edu
310-267-0170
--
My musings as a young clinical scientist: http://theclinicalbrain.blogspot.com/
Follow me on Twitter: @tully_laura

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--
--
Laura M. Tully, MA
Social Neuroscience & Psychopathology, Harvard University
Center for the Assessment and Prevention of Prodromal States, UCLA Semel Institute of Neuroscience
ltully@mednet.ucla.edu 
ltully@fas.harvard.edu
310-267-0170
--
My musings as a young clinical scientist: http://theclinicalbrain.blogspot.com/
Follow me on Twitter: @tully_laura