Difference from Previous Month

[Tim Miller]

Using Access2000
I have a Month field (formated as yyyy/mm) and a Number (currency). There
are more inconsequential fields.

I want to report this with one additional field: Difference from last
month.
In other words, I want a field that for each row reports the different
between the "number" for the current month and the "number" for the previous
month.

a: 2003/10; $ 25; [diff from last month] (in this case $10)
b: 2003/09; $15; [diff from last month] ($ 5)
c: 2003/08; $10; [diff from last month] (- 20)
d: 2003/07; $30;

etc.

Thanks for any help!