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Hi, I'm modelling a task in which participants perform the same cognitive task on a number of stimuli which differ only in their color. I'm having trouble working out the best way to model this and would love feedback. I have stimuli of four different colors, with responses to each color split into correct and incorrect responses. A. If I consider modelling the process and then the color separately [i.e., regressors (1) correct, (2) incorrect, (3-6) color], I think I'm likely to end up with a degenerate design. B. If I model correct and incorrect responses by color [(1)blue-correct, (2)blue-incorrect, (3)purple-correct...(8)] it seems intuitively wrong - I'm splitting variance for a cognitive process that is almost identical between four different regressors (for both correct and incorrect responses). I also think I lose a lot of power in each regressor's estimate. I'm sure there's a relatively easy solution; any suggestions? Many thanks Christopher