The World of Simulation

Computer simulation is a key tool at Our analysis depends on simulation models to reveal the social structures that contain the root causes of the sustainability problem. Once we've built a model of the problem using model based analysis, we can push and poke on it to see how the model and the real world it reflects behave. Mental models can go only so far, so they absolutely cannot do this for complex problems. We must instead use simulation models. Without these models we would be as lost at sea as a ship without a rudder.

The ultimate purpose of a simulation model is to allow a more accurate, reliable, and complex mental model.

Without simulation models, the mental models we rely on for all conscious decisions would be totally unable to handle the very large amounts of complexity and subtlety present in complex social system problems. It follows that without simulation modeling, problem solvers will be been unable to uncover the many insights required to solve extremely difficult complex social system problems, such as the sustainability problem.

Typical simulation model

A typical running simulation model. It runs just like any other desktop program. After you open a model and start it, sliders appear on nodes whose values can be changed while the model is running. Tiny graphs also appear on nodes whose values change dynamically. Model output is displayed in the graph at the bottom. This changes in real time as you drag a slider. Seeing a model do this is a stunning experience, because it begins to open the door to powerful new insights that are impossible to reach with any other tool.

The simulation software we have selected is easy to use. Anyone who can do a download and install can probably run the models at This is a potentially enlightening experience, because these models so clearly and simply show why the human system behaves the way it does on the sustainability problem.

And then, once you start running the Thwink models, you can take the next step. You can modify them to run differently, so that you can begin to build your own worlds and explore your own hypotheses about what may or may not be a cause or a solution to the problem. The skills to do this can be accumulated so gradually that the learning curve is no more steep than the one involved in learning the finer points of a word processor. You can learn these skills on your own or you can contact us for a helpful introduction, which can be done over the phone while you are at your computer. You can also discuss modeling on the forum. In depth training is available from the maker of the software, as well as numerous college level courses. But the real road to mastery is self study.

The model shown is the first model in the Dueling Loops paper.

How to Use Vensim to Run the Models

We have selected Vensim as our modeling software, based on its ease of use, expressive power, real time orientation, and low bug rate. This is excellent software that we can recommend very highly. Vensim uses the identical stock and flow modeling approach that was used in the Limits to Growth project.

All our models were built using this tool. The version we use is graciously available for free from Ventana Systems. To download it go to the download Vensim software page. You need the Vensim PLE version. The Users Guide is available here.

Install the Vensim software. To run a Thwink model, download the models, unzip them to a directory, double click on a model file (these have an mdl file extension) and the model will open.

Once a model is open it is easy to run. To run a model, use Control B or on the menu choose Model and then Start Synthesim. The model will start running. Little sliders under the nodes whose values can be changed will appear. To see a graph of a simulation run change right before your eyes in real time, drag a slider. The most interesting sliders to drag are the ones that affect the low and high leverage points.

When you start running a model, a dialog asking "Dataset (name) already exists. Do you want to overwrite it?" may appear. Choose Yes.

To navigate to different pages, press page up or page down. That's about all it takes to run the models.

For Further Study

We would like nothing more than to see thousands of analytical activists take up the fine art of simulation modeling. Contrary to popular belief, you do not need a college degree related to modeling. Nor do you have to be born a gifted modeler. Anyone who can think clearly and analytically can learn how to model. It is no more difficult than any other complex skill.

Step 1. The first step is an easy one: watch these short videos:

3. The Basic Concept of Feedback Loops, with Pop Growth, 9 min

4. How Simulation Models Work, with Pop Growth, 10 min

And then if you like what you've seen, take in the entire Dueling Loops Video Series. This will give you an overall feeling for the potential of modeling.

Step 2. If you want to take the second step on the royal road to modeling, probably the best place to start is Peter Senge's The Fifth Discipline: The Art and Practice of the Learning Organization, first published in 1990. This book will not teach you how to do simulation modeling. Instead, it teaches a much more important skill: systems thinking. This is the book that introduced systems thinking to the business world in the 1990s. You can read more about it at change resistance and here. This was the book that opened my eyes to what systems thinking is all about. I was not alone. The book became a best seller, selling over 500,00 copies and spawning a mini industry, several sequels, and a companion field book.

Step 3. The third step is a steep one. There is no way around it. If you sincerely want to master simulation modeling, the best book in the world is Business Dynamics: Systems Thinking and Modeling for a Complex World, by John Sterman of MIT, 2000. It's a big book. It's a long book. And it's a well written book for those with a deep thirst to find out what systems thinking and simulation modeling is all about, at the deep, high quality, real world problem level.

I've read Business Dynamics twice. The first time was a frustrating chore because there were so many new deep concepts. I felt lost, even after finishing it. But after a one year pause I read it again and night became day. The pages began to speak to me. I began to understand what Sterman was really trying to say.

Step 4. Sooner or later there is one final step that must be taken. If you put it off forever, then you will never become capable of modeling social problems well enough to gain the fresh new insights necessary to solve them. This step is the building of lots of practice models. Start modeling anything that interests you at first. And then, start modeling behaviors you don't understand, but suspect that a model could penetrate. Keep pushing until you breakthrough into the mindset of seeing the world as a gigantic dynamic system of zillions of feedback loops. In that new world, all you have to do to solve a complex social system problem is pull out your butterfly net and scoop up the handful of feedback loops that matter, walk back to your computer, and then empty them out into your model. With practice you can catch them every time.

Welcome to the world of simulation modeling, and may many happy hours and fundamental new insights come your way.

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