I think one of the problems with the limit setting that I attempted to do last week was that I was using a random number generator to decide if a particular MC soft muon candidate is tagged. In the tail of the multiplicity distribution where the yields are small, we can get incorrect systematic estimates this way due to random fluctuations. Below is a comparison of the number of events with 2 and 3 extra muons in a W+b MC sample between using and RNG and exact weighting.  The systematic yields are calculated using exact weighting while the central yields have both techniques.

The systematic variations are symmetric with respect to the exactly weighted central values while they are sometimes asymmetric with respect to the rng value. Using exact weights eliminates the false asymmetry.