Worked example

The aim of this page is to illustrate the process of converting a published model into Simile. Once a model is in Simile, we will then handle the process of getting the model onto the web. In the near future, we will be providing a direct route for getting the model onto the web, using Similette, a browser based modelling environment which is currently under development.

The model chosen for this exercise is taken from the System Zoo 3 e-book, written by Hartmut Bossel, who also wrote the models in all 3 volumes. There are some 100 models in total, about which half are directly relevant to ecological, environmental and sustainability issues. So what you learn from this exercise can be applied directly to these other models, since Hartmut had been very thorough and consistent in documenting the models.

The take-home message from this example is that converting a model into Simile is largely a mechanical exercise - copying a diagram on a one-for-one basis from one modelling environment (in this case, Vensim) to another (Simile); and entering the equation into each corresponding variable. In other words, you do not need to know "how to model" (i.e.how to make a model from scratch), but how to use Simile - a much simpler proposition. Nor do you need to understand (let alone agree with) the model you are converting - just how to read the symbols in the model.

Step 1. Read the documentation for the original model.

In fact, this is not actually necessary - you could just copy the diagram and the equations - but it will give you a sense of ownership of the process if you do know what the model is about.

The documentation for the sample model is here: Z605 Original documentation. This is the exact original chapter from Hartmut's book (he has kindly given permission for all chapters in all 3 books, provided they are made available only as the separate chapters.

Step 2 Copy the diagram into Simile

I'll give guidance on using Simile on another page. For the time being, look at these two diagrams, which show the model in the original format (Vensim), and in Simile. You can see that there is a direct, one-for-one correspondence between the two formats, despite small cosmetic differences.

There are several things to note about the Simile diagram, in relation to the original one:

• 1. The Simile diagram is all red! This is because I have shown the diagram as it appears when it is first made. I.e. just as the diagram, without any values or equations entered into the model. A red symbol shows that it still needs a value or an equation, an dthis applies to all of them at this stage. Don't worry - it will look better when the model is fleshed out in the next step.

  2. There are some small built-in cosmetic differences between Vensim and Simile. For example, Vensim does not use a symbol for variables - instead, arrows point to or from the label itself - whereas Simile has a symbol (the circle with two quadrants filled in) to signify a variable.

  3. There are also some cosmetic differences in the typographic conventions. Unlike the previous issue, these are a matter of personal choice, but they reflect the conventions that I would like you to follow in converting models. For example, I use a name beginning with a capital lletter for compartments (stocks - the rectangular boxes). Also, I do not use capital letters for all the inputs - I find that convention a bit shouty, and in any case the status of a variable can easily change from input to intermediate simply by adding or removing an influence arrow (the thin, curved arrow).

For the record, it took me less than 10 minutes to draw the diagram in Simile, copying the original one. OK, it may take you a bit longer if you are new to Simile, but not that much longer - this is pretty straightforward, and pretty similar to drawing the same diagram in Word or Powerpoint.

Step 3. Enter the values and equations for the model variables

The following table shows the equations as given in the document describing the original Vensim model, and the corresponding way these are represented in Simile. Note that you only enter the information in blue.