18        Optimization

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Optimization is supported in Vensim Professional and DSS only.

To optimize is, quite simply, to achieve the best.  Optimization can be used to validate and estimate parameters (calibration), or to select among alternative policies (policy optimization).

In order to use optimization, you will need to define what is good and what is bad — called the payoff.  The payoff is a measure, reported at the end of the simulation, stating numerically how good the simulation was.  The payoff collapses your entire model, over the entire time it was simulated, into a single number.  The payoff definition file has the extension .vpd.  A detailed description of the payoff function is contained in the Reference Manual.

After defining the payoff, you need to select which Constants to vary in order to maximize the payoff.  The selections are stored in the optimization control file, which has the extension .voc.  This file also contains information about how to optimize, and under what circumstances to stop.

At the end of an optimization, the results will be reported and are also written to a file called runname.out.  Depending on what options are selected, other files may also be created.  The file runname.out contains the values of the Constants that optimize the payoff.

NOTEOptimization can be interrupted by clicking on the Stop button or pressing the Esc key and will shut down cleanly for later resumption.