i have no particular source.....
the general consensus from users is that 2.5mg of femara does about the same as 1mg of arimidex. now, some drugs tend to work better for some users ie a user may not see the desired rusults from 1mg of adex but see great results with 2.5 or even 1.25mg of letro. (conterfiets and underdosing being out of the equation)........and vise-versa