Probability Average aggregation

Hi
I have a cube with time dimension and in measure dim i have probability element. But theoretically probability of quarter 1 is not equal to sum of probability of jan, feb, mar. It is average of jan,feb,mar. TM1 always aggregates values as sum. How do i change it to average. I have time dim with hierarchy TotalYears->(years from 1990-2011)->Quarters->Month. Same type of aggregation(Avg) must reflect to all consolidations. How can i do that. Please help me.

Thanks
Daya