How to calculate the monthly variances in the query

Hi Gurus;

I need to design a query which calculates the variance of costs with respect to the previous month. I want to select a fiscal period range and only show the variances for these selected periods. However when I enter a range the calculations for the first period dont work. Also If I add the fiscal period characteristic over the structure the formulas are not calculated for the previous period.

Need help?

How can I calculate the variance of costs with respect to previous months and display them monthly?

Thanx a lot in advance,

Duygu Sile