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infections are significantly more brief. We may vary
ratio-asy-days
to see if a
smaller length of illness for asymptomatic infected humans changes our conclusions.
Finally, the slider-set parameter
clean-likelihood
will allow us to test our main
hypothesis which that some level of individual cleaning could control an outbreak of
cholera even in the case that the local water remains contaminated. Some examples of
individual household cleaning include frequent washing of hands, household objects,
and linens, and the levels of
clean-likelihood
can reflect typical adherence to the
rules for cleaning.
4.3.2
ABM Modeling Exercises: Cholera and the NetLogo
BehaviorSpace
We begin with a series of exercises that will help us understand the behavior and
sensitivities of our model as the parameters change value. We suggest that all students
complete exercises
4.33
,
4.34
,
4.35
, and for the remaining exercises, either divide the
tasks among the class to report back to the groups as a whole, or allow students to refer
to the online file “Master Summary of Simulations.pdf” to interpret the results of the
simulations requested. These exercises are time-consuming if students are required
to perform all simulations, but otherwise take only a minute or two to consider.
Exercise 4.33.
Download our
cholera.nlogo
model from the textbook's online
materials. Observe the “Interface” tab. In addition to the slider-set parameter values
we discussed previously, you will see eight gray boxes at the bottom of these screen.
These are “Monitors” allow us to display the values of any expressions, such as the
number of agents which have a certain characteristic. Click “setup” and then “go,”
and stop the model after about 100 days have passed. How much variation do you
observe in the reported values in the gray monitor boxes as you run the simulation a
few times?
In a single simulation, the results depend on the probabilities of where each agent
moves, and the probabilities of each agent picking up, spreading, and/or becoming
infected with bacteria. While we can observe general behaviors by observing a series
of disease simulations, we cannot draw firm conclusions about specific quantitative
measures of the spread of the disease (as measured by the numbers in the gray boxes)
based on a single simulation, or even based on several simulations.
Fortunately, NetLogo has a terrific tool built in that allows us to repeat and record
multiple simulations while we step away from the computer. We will use this tool,
the BehaviorSpace, to carefully analyze our model.
Exercise 4.34.
From within the cholera model interface, click on “Tools” followed
by “BehaviorSpace,” and in the window that pops up you will see a number of experi-
ments that we have already created. Click on the top experiment, baseline experiment,
and click the “Edit” button (see Figure
4.8
a). Here is a detailed list of the options we
will use in the “Experiment” interface that pops up (see Figure
4.8
b):
In the box that follows the instructions “Vary variables as follows” we have already
listed all values that we want to hold for our baseline experiment.
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