60
4. Epidemics in the Marine System
assumes that all 10 initially infected individuals are in the first week of having the disease . To confirm this, use the QUELEM built-in function in
STELLA. This function retrieves the value of a particular slot of the conveyor. For example,
INFECTED ONE = QUELEM(INFECTED,l)
yields the value in the first slot of the conveyor named INFECTED. Set up
similar converters for the second, and third slot. Plot the values of the new
converters in a Table and see how groups of individuals move through the
conveyor. Notice also that some of these individuals disappear from the
conveyor as they "leak" through the DEATI-IS flow pipe.
Our model now looks like Figure 4.8. Run the model and you will see that
the result is virtually unchanged over the case in which the initial INFECTED
population is equally distributed across the 3-week time interval over which
they can pass on the disease. This should be no surprise because the rate of
TOTAL POPULATION
o
INFECTED ONE
INFECTED TWO
MORTALITY RATE
INFECTED THREE
DEATH
CONTACT RATE
LOSS OFIMMUNITY
IM'v1l.JNE
RECOVERY
FIGURE 4.8