Hello,
Using detail data similiar to the example below, I am trying to report the change on the Amount field from the 3rd record prior to current (4 weeks) and am struggling on a formula to acheive this. Any help is appreciated.

Item       Date           Amount     1WkChg      4WkChg
1001      01/01/09    100.00            0.00            0.00
1001      01/08/09    100.00            0.00            0.00
1001      01/15/09    200.00        100.00            0.00
1001      01/22/09    200.00            0.00        100.00
1001      01/29/09    300.00        100.00        200.00
1001      02/04/09    350.00          50.00        150.00

Each row is a weekly snapshot of the progress of the item that needs to show current value and difference in value of the same item previous week and 4 weeks ago. The first 3 weeks would produce 0 as no change and week 4 would subtract current week amount from the "3rd previous" record.