I am measuring the number of events that happen within n minute intervals for a group of 100 stores. My problem is, I am looking for a way to calculate a threshold for the acceptable number of times this event can occur within the 10 minute interval without some established boundary. Although this is a Poisson distribution can I use process control limits of 3sd? And if I use a process control does that mean my upper control limit is my threshold? And if a value exceeds how should that be interpreted?