Your understanding is correct @OriolAbril.
I think you are suggesting I change the MultiTrace object that is provided by sample(). (If not, please correct my misunderstanding.)
How can the MultiTrace object be changed? trace['mu'] provides an numpy array of the values of mu, one element in the array for each draw. But changing those values seems to have no effect on the original trace. It appears that trace['mu'] creates a new copy of the array.
