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 ===================== 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, 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] > >