1. A method of controlling an elevator system including an elevator group including several elevators and related call devices by controlling each elevator in a manner determined by calls entered and existing control instructions, comprising the steps of: performing a real time systematic decision analysis to decide between two or more alternative actions and their resulting effects, said resulting effects being estimated by simulating in real time future behavior of the entire elevator system for each alternative action, for which Monte Carlo simulation realizations are generated using randomly generated numbers for unknown quantities associated with a current state of the elevator system and for new external future event, making a control decision to use one of the alternative actions based on the systematic decision analysis, and controlling the elevator group based upon the control decision.