I've got a report that is trying to show 2 different things..

1.) a set of units sold from the day the report is run for the last 365 days sorted by part ID's. This is not the problem
2.) within the set defined in #1, the units sold in a "window" of time. Say, the first 2 weeks and the last 2 weeks of the data out of #1 Again, using the sorted by part ID's.

Getting the sum of units sold out of these for each part is not the problem. The problem is, I need to figure standard deviation on each part for both #1 and #2. #1 isn't hard to do, but trying to figure out how to do the average and other related calcs to get the standard deviation for the data in the window, seems to be a bit hard to do.... any ideas?