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First, you'd need to establish that there isn't an interaction between your covariate and your group split in the two-sample t-test. If there is no interaction, then you can have it as a single column. 0s and 1s would be sufficient.

Alternatively, if you use a full factorial design (2x2 ANOVA) you could have group and your covariate as factors and add the interaction term to the model.

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
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On Tue, May 22, 2012 at 9:13 AM, Omar Mothersill <[log in to unmask]> wrote:
Dear SPM users,

I wish to add a categorical covariate (i.e. control or patient) to a second-level analysis (a two-sample t-test).  In the covariate vector, should I list one group as 0 and the other as 1, or should I list one group as -1 and the other as 1.  Is there a more highly recommended method?

Any advice would be greatly appreciated.

Kind regards,

Omar

--
Omar Mothersill

Room 0.18
Neuropsychiatric Genetics Group
Dept. of Psychiatry
Trinity Centre for Health Sciences
St. James's Hospital
Dublin 8.

Phone: 01 896 2464
Email: [log in to unmask]