Dear Douglas, 

Thank you for your guidance. 

I am following-up on your response with regards to setting up DODS matrix to test interaction between two continous covariates. If I understand your recommendation correctly (please correct me if I am wrong), I am averegaing slopes for the new interaction covairate (age*weight), while regressing out slope estimates for age and weight. I am curious to know why regressing out the two original covariates is recommended. 

Thanks for your time! 

David 
_____________________________________________

You need to put in age, weight, AND age*weight. If you have two classes 
and use DODS, then the contrast would be
0 0 0 0 0 0 0.5 0.5
On 4/13/18 8:02 PM, David S Lee wrote:

Hi Douglas,

Thank you for your reply.

To follow up, I am setting up GLM analysis with 1 categorical variable 
and 1 continuous variable (age X weight). Myfsgd looks like this:
/GroupDescriptorFile 1/
/Title Cortical Thickness Interaction between Age and Weight/
/Class class1 plus blue/
/Class class2 circle green/
/Variable           Demean Age X Demean Weight/
/Input001Class1500/
/Input002Class2433/
/.../

I want to test for an interaction between age and weight while 
controlling forgroup. Instead ofputting-inage and weight separately, I 
multiply them to test for interaction. Can you tell me if following 
contrast vector will correctly test for this?
/0 0 0.5 0.5/
/
/
Thank you for your time,
/
/
--
David S. Lee
Associate Research Specialist
Center for Healthy Minds
University of Wisconsin - Madison
(608) 890-1115 <tel:%28608%29%20890-1115>

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