Systems Dynamics Tools

Most of these appeared as an outgrowth of the systems dynamics approach of Jay Forrester, and his DYNAMO language. Stella was one of the first ones that got a worldwide recognition due to a very nice user-friendly GUI and a wise marketing program that mostly targeted students and university professors. In the years that followed, there appeared a number of other software packages that are better than Stella in many aspects and are certainly worthwhile investigating and comparing prior to any purchase decisions. Below is a brief overview of some of the main players in this field. It should be noted that there is much development going on and new features get added to the software packages quite rapidly, so it is always recommended to check out the latest progress on the respective web pages.

• Stella - isee systems (formerly HPS), http://www. - free player and 1 month trial version, Mac/Win

Most used in the academia, and has much legacy code developed. Over the last decade has been heavily prioritizing the user interface features with nice capabilities to create game-like models, where the modeling part can be hidden from the user and only the front end similar to a flight simulator dashboard is provided. Recent addition of isee.NET Framework promises more integration with other tools, but is not extensively used and tested yet.

• Vensim - Ventana Systems, Inc., http://www. - free Vensim PLE (personal learning edition), Mac/Win

Same basic features for stock-and-flow modeling, with recent addition of some important functionality, such as calibration (will automatically adjust parameters to get the best match between model behavior and the data), optimization (efficient Powell hill-climbing algorithm searches through the parameter space looking for the largest cumulative payoff), Kalman filter, Monte Carlo analysis, Causal Tracing (a tree diagram shows a selected variable and the variables that 'cause' it to change), etc.

Vensim DLL (dynamic link library) is a way to communicate with other applications such as Visual Basic, C, C++, Excel, multimedia authoring tools, etc. The DLL allows access to a Vensim model from custom-built applications, it can send data to Vensim make changes to model parameters, and collect the simulation data for display.

• Powersim - Powersim, -free player and trial version, Win

Mostly tries to carve its niche in the business community. Communicates with Excel. Powersim Solver is a companion product that handles calibration, optimization, risk analysis and risk management.

• Madonna - UC Berkeley, - free run time version, Win/Mac

Runs many times faster than Stella. Will do parameter calibration (curve fitting) and optimization. Has several more numeric methods to solve ordinary differential equations. Stella-compatible - will take Stella equations almost as is and work with them.

• ModelMaker4 - Exeter Software (formerly Cherwell), - no free versions, Win

Same type plus quite extensive optimization and numeric methods, including Marquardt or simplex methods, simulated annealing and grid search methods ofinitial parameter estimation, ordinary, weighted, and extended least squares methods of error scaling, comprehensive statistical reporting, Monte Carlo global sensitivity with 14 distribution types, five different integration methods (Runge-Kutta, midpoint, Euler, Bulirsch-Stoer, and Gear). Gear's is an appropriate solver for stiff simulations where processes happen on very different timescales.

• Simile - Simulistics (formerly open-source AME, Agroforestry Modelling Environment), http://www. - free evaluation edition, Mac/Win/ Linux

Allows object-based representation that handles disaggregation and individual-based modeling, autogenerates C++ model code, plug-and-play modules. Supports modular modeling: any part of a model can be extracted and used separately. Has plug-in displays, allowing field-specific graphics. Also has options for spatial models with some links to geographical information systems (GIS).

The basic mathematical formalism and the interface conventions used in all these packages are quite similar. So once you have mastered one of them, it should be quite easy to switch to another one if you are looking for certain special features.

Pros. The development of all this modeling software has certainly simplified the process of building models to the extent that programming and mathematics were no longer needed to put together models. Systems dynamics became widely used in a variety of applications.

Cons. There was also the reverse side to it. Most of the software developers advertising their products will tell you that now building a model became as simple as clicking your mouse. Unfortunately, this is still not quite so and can hardly ever be so, since modeling is primarily a research process that requires knowledge and understanding of the system to generate more knowledge and more understanding. By simply putting together diagrams and pretending that now you can run a model of your system you may generate much false knowledge and illusions. The modeling systems are indeed very helpful if you know how to build models. Otherwise they become deceptive distractions.

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