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I'm currently running a group comparison with SPM's two-sample t design and I want to know how I should dummy code a categorical variable. It doesn't seem like SPM includes a constant term in the design matrix so I'm wondering if I should be using K or K-1 vectors to control for my categorical variable. For instance, if I want to control for Right, Left, and Ambidextrous would I do: G1 G2 right left ambi 1 0 1 0 0 1 0 1 0 0 1 0 1 0 0 1 0 0 1 0 1 0 0 1 0 1 0 0 1 0 1 0 0 0 1 1 0 0 0 1 0 1 0 1 0 0 1 1 0 0 0 1 1 0 0 0 1 0 0 1 0 1 0 0 1 0 1 0 1 0 0 1 0 1 0 OR G1 G2 right left 1 0 1 0 1 0 1 0 1 0 1 0 1 0 0 1 1 0 0 1 1 0 0 1 1 0 0 0 1 0 0 0 0 1 0 1 0 1 1 0 0 1 1 0 0 1 0 0 0 1 0 0 0 1 0 1 0 1 0 1 In the second example the ambidextrous group would be my reference category. Thanks! AS