15.871 Syystem Dyynamics
Recitation#1
1
A simp
ple model that cap
ptures the dyynamics
of an infectious disease‐ SARS
Historical data: SARSDATA.vdf ‐the
the incidence (rate at which new cases were reported, measured in people/day)
‐ cumulative prevalence (cumulative number of cases reported, measured in people) for SARS in Taiwan 2
Step 1: Open A New Model
3
Step 2: Initial Settings
4
Step 3: Save As “SARS
SARS_Tutorial_F12
Tutorial F12”
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Step 4: Load SARSDATA.vdf
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‐‐‐‐Warning
Warning‐‐‐‐
• We will
ill compare our siimullati
tion d
data
t to th
the acttual/hist
l/hi toriicall data in this file, so variable names need to match EXACTLY
• SARSDATA has 2 variables, those names have to match:
– New Reported Cases
– Cumullati
tive Reportted
d Cases
C
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Step 5: Population Susceptible to SARS
(
(stock)
k)
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Step 6: Population Infected with SARS
(
(stock/level)
k/l l)
9
Step 7: Infection Rate (flow/rate)
First click in center of this stock
Then click in center of
this stock
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Step 8: Infectivity (auxilary)
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Step 9: Add Causal Arrow
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Step 10: Right‐click on Arrow “handle”
to Add
dd Polarity
l
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Step 11: Contacts Between Infected
and
d Uninffected
d Peoplle
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Step 12: Complete the Rest
off th
he Model
d l + Lab
bell the
h Loops
15
Use comment tool to add labels
16
Step 13: Specify Equations
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Exogeneous and Endogeneous
Variabl
bles
• Variables
a ab es in SD
S models
ode s aaree classified
c ass ed as either
et e exogenous or endogenous:
– Exogenous variables are defined independent of other variables of the model. • They are functions of time (i.e., Exogenous Variable = f(t)). • Of course the exogenous variables may be constants, in which case they are called parameters. – Endogenous variables are influenced by other
variables in the system
system
• Endogenous Variable = f(x, y, z), where x, y, z are other variables in the model
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Step 14: Define Infection Rate
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Step 15: Define Population Susceptible
to SARS
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Step 16: Define Susceptible Contacts
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Step 17: Define Contacts Between
Inffected
d and
d Uninffected
d Peoplle
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Step 18: Define Probability of Contact
with
h Inffected
d Person
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Step 19: Define Total Population
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Step 20: Define Contact Frequency
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Step 21: Define Infectivity
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Step 22: Define Population Infected
with
h SARS
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Time Out and Run
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Plotting Results
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Optional: Permanent Graph
Use: I//O Ob
bject or Custom Graph
h
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Step 23: Add Variables in the Dataset
31
Step 24: Add Infection Rate
( a shadow
(as
h d variable)
bl )
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Step 25: Add Causal Link and Polarity
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Step 26: Define New Reported Cases
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Step 27: Define Cumulative Reported
Cases
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Step 28: Units Check
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Step 29: Error!!!
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Step 30: Redefine Contact Frequency
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Step 31: Units Check – A. O. K.
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Step 32: Rename Case – “Base” and
Run
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Step 33: Plot Cumulative Reported
Cases
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Step 34: Try Lowering Contact
Frequency
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Step 35: Change Run Name
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Step 36: Displaying Multiple Runs
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Step 37: Synthesim
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Amazing!!!
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MIT OpenCourseWare
http://ocw.mit.edu
15.871 Introduction to System Dynamics
Fall 2013
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