I have done a little Maple study on efficiency concerning the a-parameter. The purpose of the exercise is to get a feeling for the systematics. http://www.slac.stanford.edu/~elsen/efficiency.pdf I have tried to model the efficiency dependence according to the BAD. If I fold that distribution with two Mx distributions that roughly agree with what is caused by the variation of the a-parameter I can study the variation of the number of events below 1.55 in Mx. The original model distributions differ by <7%. After folding with the efficiency the effect translates into a 20% variation in the number of events and hence on the branching ratio. Surprisingly the largest variation is at the rho-peak. It is thus important that in all these studies the differential efficiency is properly applied. Incidentally do you have the efficiency distribution for the non-resonant model? I am worried that what is called efficiency may be a convolution of efficiency and resolution. Eckhard