An operations analyst is developing a model of the checkout process at a grocery store. In his model customers arrive to the check-out process one at a time and pick the lane with the shortest line. The time between customer arrivals follows an exponential distribution. The time to process the checkout follows a lognormal distribution. When a transaction is completed that customer leaves and the next customer begins service. The analyst wants to build an anlytical model of this process in order to observe how lines form and determine when additional lanes should be opened. The most appropriate form of model is