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Dear All: Urgent help is needed. I have a data set in matrix format of three columns: X, Y and index of four groups (1,2,3,4). What I need to do is the following; 1- How I can subtract the sample mean of each group indexed 1,2,3,4 from the corresponding data values of this group and create new columns say X-sample mean and Y-sample mean? I tried to use the "tapply" but