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4 Using an Existing Model
| • | In the Vensim Development Environment, click on the Open Model button on the Main Toolbar or select File>Open Model. |
| • | Choose the \bpr directory, then open the model bpr3.mdl. |
| • | Spend some time looking over the structure of the model. This model has only one view (in the sketch window), many models contain more than one view. If you have a large screen, change the menu item View>Zoom to larger than 100% to make the view more readable. |
| • | Examine the model structure with the Causes Tree diagram (the top analysis tool). Note how the Tree diagram shows causal relationships between variables; follow the same relationships in the sketch view. Any variable appearing in brackets ( ) indicates a feedback loop. Now use the right mouse button to click on the Causes Tree tool. A dialog box opens. Check the box Activate on variable selection and click OK. Now double click on any variable in the tree and another tree diagram is automatically generated. Continue tracing links throughout the model. If a tree stops at a constant, select a variable on a different tree, or choose a new variable by selecting the menu item Windows>Control Panel then clicking the Variable tab and selecting from the list. When you are finished, click with the right mouse button on the Causes Tree tool, uncheck the box Activate on variable selection and click OK. Select the menu item Windows>Close All Output. |
| • | Double click on the runname editing box on the Main Toolbar, type in a new name for the run (e.g. bpr1) |
| • | Click on the Simulate button. |
| • | A work-in-progress graph will be generated showing orders in different states. Now you have a model, a dataset, and an output graph. Close the graph by clicking on the top left corner (or choose the menu item Windows>Close All Output). Click the Control Panel button on the Main Toolbar then click the Variable tab. Choose the variable activating from the Variable Selection control. Find this variable in the sketch by first clicking on the sketch, then selecting menu item Edit>Find Workbench. The variable will now be highlighted in your sketch view. |
| • | Click on the Causes Strip tool. Note how the oscillating behavior of activating is generated from the oscillations in Activating Capacity and Awaiting Activation. |
| • | Double click on Awaiting Activation (in the graph or in the sketch view), then click on the Causes Strip tool. Note how the oscillating behavior of Awaiting Activation is generated from the oscillations in activating and testing. The behavior of dispatching is not contributing to the oscillations in Awaiting Activation. |
| • | Double click on the variable dispatching then click on the Causes Strip tool. Observe how the behavior of dispatching mirrors the behavior of dispatching capacity and does not closely follow the behavior of Orders Requiring Service. |
| • | Continue examining this model with causal strip graphs. Try using other analysis tools; the first four tools examine the structure of a model, most of the other tools examine simulation datasets to determine behavior of variables in the model. |
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