Comparison to previous date with date as dimension

[#1]

Hello,

Ineed help to include comparisons  to previous weeks into my PivotTable.

My dimensions is the date and my expression is the sum of a numerical variable for each date.

I would like to add a second column that contains the sum of the numerical variable for the week before

I.e.,

Date   SUM(variable)   SUM(variable) - 7 d            

2012-02-01   10                 8

...

2012-02-08   12                10

Thanks,David

[CELAMBARASAN]

Hi,

     you cann't use variable in the sum function if it has value .has expression if so what expression?

Celambarasan