I've built a matrix report showing the total sales of a given product with respect to the sales year and month which are the column fields of the report. The function aggregate sum is very powerful in determining the running total with respect to a given scope. However, my challenge is to find the difference between sequential and adjoining columns of data for a given scope. i.e. what is the change in total sales from month to month? I've searched endlessly for information regarding and believe that this seeming basic question has to have been answered before. Any ideas?