Thanks very much for both your suggestions. When you refer to doing a 'double for loop', do you mean finding the average for rowgrp and colgrp within each 6x6 block? If so, how would this be done so that the whole data frame is covered? It would seem to me that the 'mean' operation would need to combine rowgrp AND colgrp together? How would I get the loop to cycle through each set of 6x6 blocks of the data frame?
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