Standard Deviation in Cube

[nakisoen]

Hi there,
I need help with the following.

I was asked to implement Standard Deviation in a (web)cube.
What is the best way to do so?

Period has to be variable at all times, so I have to generate the average (mean) at the report-side and not at the cube site, I guess? But then, how can I substract this average, from each value in that period?
The values are at a lowe level, so I get stucked at this poitn..

Please help...

Many thanks!
Navin