24
2. Modeling in STELLA
After you specify REPRODUCTION RATE = .05 click on OK. At the right of
the STELLA toolbox is the connectorGnformation arrow). We use the connector to pass on information (about the state, control, or transforming variable)
to a control or transforming variable. In this case, we want to pass on information about the REPRODUCTION RATE to REPRODUCTION and about the
size of the stock WHALES to REPRODUCTION. Both the size of the stock at a
point in time and the rate of reproduction influence the inflow of births.
Once you drew information arrows from REPRODUCTION RATE and
from WHALES to the control, open the control by clicking on it. Recognize
that REPRODUCTION RATE and WHALES are listed as the "required inputs" for the specification of REPRODUCTION. Notice also that STELLA
asks you to specify the control :
REPRODUCTION = . .. "Place right hand side of equation here. " (1)
Click on REPRODUCTION, then on the multiplication sign in the dialog
box, and then on WHALES to generate the equation:
REPRODUCTION =REPRODUCTION RATE· WHALES.
(2)
Click on OK and the question mark in the control REPRODUCTION disappears. Your model should now look like Figure 2.6.
Next we set up the temporal (time) parameters of the model. These are
the 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 dialog box will appear in which you
can specify, among other things, the length of the simulation, the DT, and
the units of time. We arbitrarily choose DT= .25, length of time = 40, and
units of time = years. When we run the model, the state variables are then
updated four times a year (each quarter of a year) and run over 40 simulated years. More detail on how STELLA does the calculation follows below
and in Section 2.3.
WHALES
REPRODUCTION RATE
FIGURE 2.6
2. Modeling in STELLA
After you specify REPRODUCTION RATE = .05 click on OK. At the right of
the STELLA toolbox is the connectorGnformation arrow). We use the connector to pass on information (about the state, control, or transforming variable)
to a control or transforming variable. In this case, we want to pass on information about the REPRODUCTION RATE to REPRODUCTION and about the
size of the stock WHALES to REPRODUCTION. Both the size of the stock at a
point in time and the rate of reproduction influence the inflow of births.
Once you drew information arrows from REPRODUCTION RATE and
from WHALES to the control, open the control by clicking on it. Recognize
that REPRODUCTION RATE and WHALES are listed as the "required inputs" for the specification of REPRODUCTION. Notice also that STELLA
asks you to specify the control :
REPRODUCTION = . .. "Place right hand side of equation here. " (1)
Click on REPRODUCTION, then on the multiplication sign in the dialog
box, and then on WHALES to generate the equation:
REPRODUCTION =REPRODUCTION RATE· WHALES.
(2)
Click on OK and the question mark in the control REPRODUCTION disappears. Your model should now look like Figure 2.6.
Next we set up the temporal (time) parameters of the model. These are
the 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 dialog box will appear in which you
can specify, among other things, the length of the simulation, the DT, and
the units of time. We arbitrarily choose DT= .25, length of time = 40, and
units of time = years. When we run the model, the state variables are then
updated four times a year (each quarter of a year) and run over 40 simulated years. More detail on how STELLA does the calculation follows below
and in Section 2.3.
WHALES
REPRODUCTION RATE
FIGURE 2.6