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Dear SPMers, Could anyone tell me what is the formula to calculate the contrast vector for a 3-way interaction? Let's say I have factors Ax1, Ay1, Ax2, Ay2, Bx1, Bx2, By1, By2. The interaction between the first and second factor is easy to calculate (Ax-Ay)-(Bx-By) which is equivalent to the vector [1 -1 -1 1]. But how do I work out the interaction among the three factors? Thank you in advance, Angela