usage probability and alternate BOM

Hi,

please tell me how to create alternate BOM ,a nd where to maintain usage probability. let say example A is the finished product and it has child items C, D.

Now i want to make child E as as alternative to C. How to maintain in BOM.

I want to give usage probability of child item E as 60 % and C as 40 %.

so please tell me how to maintain the data in BOM