>SQL 2012 makes it quite easy:

GREAT query...that's a great example of the new ROWS UNBOUNDED PRECEDING to get cumulative sums.

I've used this as a training example, though I also use CROSS APPLY for situations before SQL 2012. I'll wait to see if anyone wants to try an example pre-SQL 2012, and then I'll post some solutions (with yours as one of them).

Thanks, Naomi,