Rangesum(above(())) special case

[Cathalc]

Hi all,

I have a special case using the rangesum(above()) expression. The table below shows the data I'm working with:

Please ignore the third column in this image. I want to count the number of defects (column 5) per monthyear and per 'ATA Chapter' (column 4). After counting these defects I use rangesum(above()) to get a running sum over 3 months. In order to make sure that the sum doesn't reset after the 12th month I included TOTAL in the expression: rangesum(above(Total )). 

BUT,

I would like the count to reset at the end of a chapter (e.g. ATA 00 to ATA 01). Is there a way to do this without also resetting thecount after the end of the year? I've been experimenting with different sorting options but nothing works. Perhaps there's an extra parameter I can put in the expression?

Thanks!

Cathal