header logo

Specifying the Input

Step 2 Rate Equation

Now, we need to define a formula for the rate Get Sick. Assume that every susceptible person interacts with every infected person once per day. This can be expressed mathematically as Susceptible People * Infected People. Not every interaction results in an infection though, so multiply this total number of interactions by the Infection Fraction to get the number of people who get sick.


  • ICON With the Equations button still selected, click on Get Sick.
  • Notice that the other variables in the system are listed in the box on the right of the input window. Because you connected these variables to this rate, they must be used in the formula for this rate. Add them to your formula by clicking once on them.
  • Enter the following formula:

    Susceptible People * Infected People * Infection Fraction

  • Leaving the units blank for now, click OK to finish.

Previous Next

Vensim PLE is a product of Ventana Systems, Inc.