Display difference current week to prior week

Question

Hi everyone,
I have following question. I need to display in a Report the difference from the current week to the prior week. I need something like this:
weeks       |      quantity      |    difference to prior week
30          |     1000           |
31          |      500           |     -500
32          |      800           |      300
33          |     1200           |      400
34          |     1000           |     -200
By starting the report the user enters the "start_date" and "end_date".
For this example the user enters from "19.07.2009" to "22.08.2009"
Do you have any ideas how to solve this?
Many thanks in advance
Mario

Answer 1

Hi Salah,
many thanks for your answer but this doesn't work on my side because each week is a summary of many different records from the database. If I work with "previous" it only subtracts the last quantity from the weeks summary, not the last weeks summary from this weeks summary.
Is it possible to change the formula to something like this:
sum ({quantity}, {@calendarweek} ) - previous (sum ({quantity}, {@calendarweek} )
something like this whould help me I think but I receive an error from the "previous" too much parameters
Now I tried to create a new formula like this:
@quantity_calenderweek
= sum ({quantity}, {@calendarweek} )
and used it with my previous formula:
{@quantity_calenderweek} - previous ({@quantity_calenderweek})
but now crystal sais that this field hast no prior or following value...