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A F-test is non-directional and will give you both directions. A T-test only gives you the direction that you test. You could also use [0 0 -1 1] as a t-test to test the other direction.

An F-contrast for your model will be a single row [0 0 1 -1]. This is the same as an F-test using [0 0 -1 1].

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 Tue, Jun 30, 2015 at 11:25 PM, xiangbo_2010 <[log in to unmask]> wrote:
Hi Donald,
 
thank you for giving the advice, I Seen somebody made the constrast using F 
0 0 -0.5 0.5
0 0 0.5 -0.5
what is different between T and F?
thank you!
 
Bo


At 2015-07-01 09:33:39, "MCLAREN, Donald" <[log in to unmask]> wrote:
Use a two-sample t-test with a covariate that interacts with factor 1.

Contrast should be [0 0 1 -1].

This will test for the difference in slopes.

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
=====================
This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED
HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is
intended only for the use of the individual or entity named above. If the
reader of the e-mail is not the intended recipient or the employee or agent
responsible for delivering it to the intended recipient, you are hereby
notified that you are in possession of confidential and privileged
information. Any unauthorized use, disclosure, copying or the taking of any
action in reliance on the contents of this information is strictly
prohibited and may be unlawful. If you have received this e-mail
unintentionally, please immediately notify the sender via telephone at (773)
406-2464 or email.

On Tue, Jun 30, 2015 at 8:44 PM, xiangbo_2010 <[log in to unmask]> wrote:

Hi everyone,

I want to perform an interaction between age and diagnosis (case and control) on the volume of gray matter, should I how to do it? and how to design contrast? thank you so much!

Bo