Simulating models is traditionally done as a batch process. Set up some assumptions, make some a simulation, and then look at some results. Twenty five years ago, this was a necessity. Computers were large, expensive, and very slow by today's standards. Access was through a time shared terminal or perhaps even punch cards and jobs had to be submitted for execution and the output picked up from the main computer office or, if you were lucky, a close at hand printer or even your own teletype terminal.
Technology has changed a great deal, but the batch processing of simulation has remained a legacy till now. While it has been possible to view the results of simulations as they progress, this has not really altered the change, compute, review approach to simulation. With Vensim 5 a better approach to interacting with models has finally arrived, and it is practical for a surprisingly large fraction of the models in use. We call this approach SyntheSim.