Database Reference
In-Depth Information
number that can be represented as a transforming variable. A transforming variable
is expressed as a
converter
, the circle that is second from the right in the STELLA
toolbox. (So far GETTING SICK is a constant; later the model will allow this rate to
vary.) The same clicking and dragging technique that brought the stock to the screen
will bring up the circle. Open the converter and enter the number of 0.05 (5/100).
Down the side of the dialogue box is an impressive list of built-in functions that are
useful for more elaborate models. We'll use some of these built-in functions later on
in the topic.
At the right of the STELLA toolbox is the
connector
(information arrow). Use the
connector to pass on information (about the state, control, or transforming variable)
to a circle, to the control (the transforming variable). In this case, you want to pass
on information about the CONTAGION RATE to GETTING SICK. Once you draw
the information arrow from the transforming variable CONTAGION RATE to the
control and from the stock SICK to the control, open the control by double-clicking
on it. Recognize that CONTAGION RATE and SICK are two required inputs for the
specification of GETTING SICK. Note also that STELLA asks you to specify the
control: GETTING SICK = . . . “Place right-hand side of equation here.”
Click on CONTAGION RATE, then on the multiplication sign in the dialogue
box, and then on SICK to generate the equation
GETTING SICK
=
CONTAGION RATE
∗
SICK
(1.1)
Click on OK, and the question mark in the control GETTING SICK disappears.
Your STELLA diagram should now look like Figure 1.8:
SICK
GETTING SICK
CONTAGION RATE
Fig. 1.8
Next, set the temporal (time) parameters of the model. These are DT (the time
step over which the stock variables are updated) and the total time length of a model
run. Go to the RUN pull-down menu on the menu bar and select Time Specs. A
dialogue box will appear in which you can specify, among other things, the length
of the simulation, the DT, and the units of time. For this model, choose DT = 1,
length of time = 20, and units of time = days.
To display the results of the model, click on the graph icon and drag it to the
diagram. If you wanted to, you could display these results in a table by choosing
