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Specifying the Input

Step 1 Box Variables

Recall that our hypothetical school has 100 kids. Set the initial number of susceptible people to 99 and set the number of infected people to 1.


  • ICON Click on the Equations button on the toolbar.
  • Every object that now appears in black is not sufficiently defined.
  • Click on Susceptible People and a window like the one below will pop up. This probably looks complex, but you won't need most of these options at first.
  • The cursor should already be in the box to the right of the words Init Value. Here, type in 99.
  • Next, click in the box to the right of Units and type people.
  • Click OK in the bottom left of the window to finish.

  • The procedure for setting the initial number of infected people is analagous, except of course that there is only 1 infected person at time zero.

Input Window

Input Window

NOTICE: It may seem as though labeling the units is redundant here, but labeling units this way will enable you to use Vensim PLE's Unit Checker, a very useful tool for catching inconsistencies in your model.

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